rekahsoft-gnu: Patch terraform sources to read GUIX_TERRAFORM_PLUGIN_DIR variable

Additionally, propagate the GUIX_TERRAFORM_PLUGIN_DIR variable so that only
guix installed terraform providers can be used. This has the effect of
disabling automatic fetching of terraform providers, which may be inconvenient
to those who are not expecting this behavior. On Nix, this is resolved by not
using a shim shell script that sets this variable when no terraform providers
are installed. This approach doesn't work very well on guix, so in the future
separate packages which inherit from the existing terraform packages but do
not set this environment variable may be provided.

rekahsoft-gnu/packages/patches/terraform-provider-path.patch

@@ -0,0 +1,16 @@
+diff --git a/command/init.go b/command/init.go
+index 403ca245b..05d98329a 100644
+--- a/command/init.go
++++ b/command/init.go
+@@ -64,6 +64,11 @@ func (c *InitCommand) Run(args []string) int {
+ 		return 1
+ 	}
+ 
++	val, ok := os.LookupEnv("GUIX_TERRAFORM_PLUGIN_DIR")
++	if ok {
++		flagPluginPath = append(flagPluginPath, val)
++	}
++
+ 	if len(flagPluginPath) > 0 {
+ 		c.pluginPath = flagPluginPath
+ 		c.getPlugins = false

rekahsoft-gnu/packages/terraform.scm

@@ -18,6 +18,7 @@
 
 (define-module (rekahsoft-gnu packages terraform)
   #:use-module ((guix licenses) #:prefix license:)
+  #:use-module (gnu packages)
   #:use-module (gnu packages golang)
   #:use-module (guix utils)
   #:use-module (guix packages)
@@ -27,6 +28,14 @@
   #:use-module (guix build-system trivial)
   #:use-module (rekahsoft-gnu packages golang))
 
+(define (search-rekahsoft-gnu-patches patch)
+  (parameterize
+      ((%patch-path
+        (map (lambda (directory)
+               (string-append directory "/rekahsoft-gnu/packages/patches"))
+             %load-path)))
+    (search-patches patch)))
+
 ;; TODO: Uses vendored dependencies
 (define-public terraform-0.13
   (package
@@ -37,6 +46,7 @@
               (uri (git-reference
                     (url "https://github.com/hashicorp/terraform")
                     (commit (string-append "v" version))))
+              (patches (search-rekahsoft-gnu-patches "terraform-provider-path.patch"))
               (file-name (git-file-name name version))
               (sha256
                (base32
@@ -53,7 +63,11 @@
             (variable "SSL_CERT_FILE")
             (file-type 'regular)
             (separator #f)                        ;single entry
-            (files '("etc/ssl/certs/ca-certificates.crt")))))
+            (files '("etc/ssl/certs/ca-certificates.crt")))
+           (search-path-specification
+            (variable "GUIX_TERRAFORM_PLUGIN_DIR")
+            (separator #f)                        ;single entry
+            (files '("libexec/terraform")))))
     (arguments
      `(#:import-path "github.com/hashicorp/terraform"
        #:install-source? #f
@@ -106,6 +120,7 @@ popular service providers as well as custom in-house solutions.")
               (uri (git-reference
                     (url "https://github.com/hashicorp/terraform")
                     (commit (string-append "v" version))))
+              (patches (search-rekahsoft-gnu-patches "terraform-provider-path.patch"))
               (file-name (git-file-name name version))
               (sha256
                (base32
@@ -120,6 +135,7 @@ popular service providers as well as custom in-house solutions.")
               (uri (git-reference
                     (url "https://github.com/hashicorp/terraform")
                     (commit (string-append "v" version))))
+              (patches (search-rekahsoft-gnu-patches "terraform-provider-path.patch"))
               (file-name (git-file-name name version))
               (sha256
                (base32