install: Adjust for (package-source guix) not being a <channel>.

Fixes guix/guix#422.

* gnu/system/install.scm (%installation-services)[guix-package-commit]:
New procedure.
<guix-configuration>: Use it instead of ‘channel-commit’.

Change-Id: I6ed5bc3f21a375e639fabfcbcdaf70b6347d1c5c

gnu/system/install.scm

@@ -33,6 +33,7 @@
   #:use-module (guix store)
   #:use-module (guix modules)
   #:use-module ((guix packages) #:select (package-version supported-package?))
+  #:autoload   (guix channels) (channel? channel-commit)
   #:use-module (guix platform)
   #:use-module (guix utils)
   #:use-module (guix packages)
@@ -353,6 +354,15 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
     (define bare-bones-os
       (load "examples/bare-bones.tmpl"))
 
+    (define (guix-package-commit guix)
+      ;; Extract the commit of the GUIX package.
+      (match (package-source guix)
+        ((? channel? source)
+         (channel-commit source))
+        (_
+         (apply (lambda* (#:key commit #:allow-other-keys) commit)
+                (package-arguments guix)))))
+
     (append
      ;; Generic services
      (list (service virtual-terminal-service-type)
@@ -403,8 +413,7 @@ Access documentation at any time by pressing Alt-F2.\x1b[0m
                                ;; Do not leak the local checkout URL.
                                (source (channel
                                         (inherit %default-guix-channel)
-                                        (commit (channel-commit
+                                        (commit (guix-package-commit guix)))))))))
-                                                 (package-source guix))))))))))
 
            ;; Start udev so that useful device nodes are available.
            ;; Use device-mapper rules for cryptsetup & co; enable the CRDA for