pull: Add GUIX_PULL_URL environment variable.

* guix/scripts/pull.scm (%repository-url): Add GUIX_PULL_URL environment
  variable.
* doc/guix.texi (Invoking guix pull): Mention GUIX_PULL_URL.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -2532,8 +2532,10 @@ Produce verbose output, writing build logs to the standard error output.
 @item --url=@var{url}
 Download Guix from the Git repository at @var{url}.
+@vindex GUIX_PULL_URL
 By default, the source is taken from its canonical Git repository at
-@code{gnu.org}, for the stable branch of Guix.
+@code{gnu.org}, for the stable branch of Guix.  To use a different source,
+set the @code{GUIX_PULL_URL} environment variable.
 @item --commit=@var{commit}
 Deploy @var{commit}, a valid Git commit ID represented as a hexadecimal

guix/scripts/pull.scm

@@ -82,7 +82,7 @@ (define (ensure-guile-git!)
                (resolve-interface '(git))))
 
 (define %repository-url
-  "https://git.savannah.gnu.org/git/guix.git")
+  (or (getenv "GUIX_PULL_URL") "https://git.savannah.gnu.org/git/guix.git"))
 
 
 ;;;