desktop: Include /etc/guix/channels.scm, referencing latest channels

* .guix/rekahsoft/guix-config/desktop.scm: Add a /etc/guix/channels.scm file so that
users (including root) that do not have their own ~/.config/guix/channels.scm file default to
channels used by this repository. This enables `guix time-machine -- describe -f` to output
the latest channels without specifying the `channels-latest.scm` file to time-machine.
* channels-latest.scm: Channel file that exactly matches `./channels.scm`, except that it
does not pin any channel to a commit, so that the latest version will be pulled.

.guix/rekahsoft/guix-config/desktop.scm

@@ -38,6 +38,9 @@
                    (listen-tcp? #f)))
 	 (service docker-service-type)
 
+         (simple-service 'system-guix-channels etc-service-type
+                         (list `("guix/channels.scm" ,(local-file "../../../channels-latest.scm"))))
+
          (service qemu-binfmt-service-type
                   (qemu-binfmt-configuration
                    (platforms (lookup-qemu-platforms "arm" "aarch64"))))

README.org

@@ -104,9 +104,11 @@ When on my network, I pull from my internal git (notice the different URL).
   curl -O https://git.home.rekahsoft.ca/rekahsoft-public/guix-machines/raw/branch/master/channels.scm
 #+end_src
 
-Once the channel file is available on the target, update guix to use these channels.
+Once the channel file is available on the target, refresh the cached command paths with
+~hash~ and update guix to use the new channels.
 
 #+begin_src shell
+  hash guix
   sudo -i guix pull -C $(realpath channels.scm)
 #+end_src
 
@@ -164,10 +166,20 @@ TODO: disks need to be partitioned and appropriately mounted. This varies depend
   btrfs subvolume create /mnt/gnu/store
   btrfs subvolume create /mnt/home
   btrfs subvolume create /mnt/var
+
+  # Create and activate swap
+  mkswap --label swap /dev/vg0/swap
+  swapon /dev/vg0/swap
+
+  # Prepare /mnt for Guix installation
+  mkdir -p /mnt/boot/efi
+  mount /dev/nvme0n1p1 /mnt/boot/efi
 #+end_src
 
 *** Bootstrap System
 
+**** Start ~cow-store~ Service
+
 As described in guix documentation, start ~cow-store~ to allow later steps to write store
 changes to ~/mnt/gnu/store~.
 
@@ -175,6 +187,33 @@ changes to ~/mnt/gnu/store~.
   herd start cow-store /mnt
 #+end_src
 
+**** Optional (but recommended): Use additional or alternative substitute servers
+
+Optionally additional or alternative substitute servers can be setup for guix-daemon. This is
+recommended to greatly speed up installation (so that nonguix and rekahsoft-guix (internal
+network only) substitutes can be used).
+
+First stop the running ~guix-daemon~.
+
+#+begin_src bash
+  herd stop guix-daemon
+#+end_src
+
+Then start a new instance of ~guix-daemon~ as it is normally started by its Shepherd service,
+but with additional or alternative substitute servers provided.
+
+#+begin_src bash
+  guix-daemon \
+    --build-users-group guixbuild \
+    --max-silent-time 0 \
+    --timeout 0 \
+    --log-compression gzip \
+    --discover=no \
+    --substitute-urls https://substitutes.nonguix.org https://guix-ci.home.rekahsoft.ca https://ci.guix.gnu.org https://bordeaux.guix.gnu.org &
+#+end_src
+
+**** Create and Bootstrap System
+
 Create a ~bootstrap.scm~ file like this:
 
 #+begin_src scheme
@@ -187,18 +226,6 @@ Use ~guix system init ...~ to instantiate the system.
   guix system init bootstrap.scm /mnt
 #+end_src
 
-**** TODO This doesn't work unless I use ~guix time-machine ...~
-
-I have to use ~guix time-machine ...~ with the channel file for ~guix system init ...~ to
-work. It fails to find the sources, complaining about a missing public interface. However, in
-repl this also fails. Even more confusingly, when using ~guix time-machine -C channels.scm --
-repl~, it also cannot find the modules from my channel. I need to ask about this on the
-mailing list. What works (no pull needed):
-
-#+begin_src bash
-  guix time-machine -C channels.scm -- system init bootstrap.scm /mnt
-#+end_src
-
 ** TODO Producing an image to be flashed directly
 
 TODO: there are limitations on what images I can produce; namely, lvm and luks cannot be

channels-latest.scm

@@ -0,0 +1,36 @@
+(list (channel
+        (name 'guix)
+        (url "https://git.savannah.gnu.org/git/guix.git")
+        (branch "master")
+        (introduction
+          (make-channel-introduction
+            "9edb3f66fd807b096b48283debdcddccfea34bad"
+            (openpgp-fingerprint
+              "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"))))
+      (channel
+        (name 'nonguix)
+        (url "https://gitlab.com/nonguix/nonguix")
+        (branch "master")
+        (introduction
+          (make-channel-introduction
+            "897c1a470da759236cc11798f4e0a5f7d4d59fbc"
+            (openpgp-fingerprint
+              "2A39 3FFF 68F4 EF7A 3D29  12AF 6F51 20A0 22FB B2D5"))))
+      (channel
+        (name 'rekahsoft-guix)
+        (url "https://git.rekahsoft.ca/rekahsoft/rekahsoft-guix.git")
+        (branch "master")
+        (introduction
+          (make-channel-introduction
+            "d910ac4ca1e1a115934b10921bc23f50bd481043"
+            (openpgp-fingerprint
+              "F8D5 46F3 AF37 EF53 D1B6  48BE 7B4D EB93 212B 3022"))))
+      (channel
+       (name 'guix-machines)
+       (url "https://git.home.rekahsoft.ca/rekahsoft-public/guix-machines.git")
+       (branch "master")
+       (introduction
+        (make-channel-introduction
+         "acaa20d632da16937508a45ef2cc0083e09bae4c"
+         (openpgp-fingerprint
+          "F8D5 46F3 AF37 EF53 D1B6  48BE 7B4D EB93 212B 3022")))))