etc: pre-push: Do not use recent ;& case statement terminator.

Some systems are still using a shell that predates its inclusion in the POSIX
specification, such as Debian Bookworm.

* etc/git/pre-push (z40): Remove usage of ';&'.  Fail when attempting to push
to Savannah instead.

Reported-by: Leo Famulari <leo@famulari.name>
Closes: #64
Change-Id: I2dd224b7a3236f7027fc702b96e3ee1ff594d4fa

etc/git/pre-push

@@ -38,7 +38,10 @@ do
 
                 # Only use the hook when pushing to upstream.
                 case "$2" in
-                    *.gnu.org*) ;&
+                    *.gnu.org*)
+                        echo "adjust remote: repo was migrated to codeberg"
+                        exit 127
+                        ;;
                     *codeberg.org/guix/*)
                         set -e
                         guix git authenticate