guix build: Gracefully handle invalid '--with-git-url' specs.

* guix/scripts/build.scm (transform-package-source-git-url): Add case
for when 'string-split' does not return exactly two elements.

guix/scripts/build.scm

@@ -370,7 +370,10 @@ a checkout of the Git repository at the given URL."
                       (package
                         (inherit old)
                         (source (git-checkout (url url)
-                                              (recursive? #t)))))))))
+                                              (recursive? #t)))))))
+             (_
+              (leave (G_ "~a: invalid Git URL replacement specification~%")
+                     spec))))
          replacement-specs))
 
   (define rewrite