environment: Pass 'git-predicate' the right directory.

* environment.scm: Use (current-source-location) instead of "." as the
basis of the current directory name.

environment.scm

@@ -38,7 +38,10 @@
   (name "guix-kernel")
   (version "0.0.1")
   (source (local-file "." "guix-kernel"
-                      #:select? (git-predicate (getcwd))
+                      #:select? (git-predicate
+                                 (dirname (assoc-ref
+                                           (current-source-location)
+                                           'filename)))
                       #:recursive? #t))
   (build-system gnu-build-system)
   (arguments