Use Guix: Pin dependencies

It was discovered that builds no longer were functional on my current version of
Guix (25b83bd). That is, builds would fail indicating 'avr-ld: cannot find crtatmega32u4.o:
No such file or directory'. avr-toolchain has been updated a few times, and an old version is
required to build this; as such, dependencies are now pinned with a channels.scm file, and an
.envrc has been provided for convenience.

A latter commit should merge qmk_firmware upstream, which likely will resolve the build issue
with the latest version of avr-gcc.

.envrc

@@ -0,0 +1,10 @@
+use_guix-shell() {
+  CHANNEL_FILE=channels.scm
+  if [ -f $CHANNEL_FILE ]; then
+    eval "$(guix time-machine -C $CHANNEL_FILE -- shell "$@" --search-paths)"
+  else
+    eval "$(guix shell "$@" --search-paths)"
+  fi
+}
+
+use guix-shell avr-toolchain avrdude make python-wrapper

channels.scm

@@ -0,0 +1,11 @@
+(list (channel
+       (name 'guix)
+       (url "https://git.savannah.gnu.org/git/guix.git")
+       (branch "master")
+       (commit
+        "273709932fa47f6ae56c96564a9415fdbff4a169")
+       (introduction
+        (make-channel-introduction
+         "9edb3f66fd807b096b48283debdcddccfea34bad"
+         (openpgp-fingerprint
+          "BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA")))))