guix build: '--with-branch' strips slashes from the version string.

This fixes things like:

  guix build glibc \
    --with-git-url=glibc=git://sourceware.org/git/glibc.git \
    --with-branch=glibc=release/2.25/master

whereby slashes would before go straight to the 'version' field, leading
to an invalid store file name.

* guix/scripts/build.scm (transform-package-source-branch)[replace]:
Replace slashes with hyphens in BRANCH when building the version
string.

guix/scripts/build.scm

@@ -318,7 +318,10 @@ strings like \"guile-next=stable-3.0\" meaning that packages are built using
   (define (replace old url branch)
     (package
       (inherit old)
-      (version (string-append "git." branch))
+      (version (string-append "git." (string-map (match-lambda
+                                                   (#\/ #\-)
+                                                   (chr chr))
+                                                 branch)))
       (source (git-checkout (url url) (branch branch)
                             (recursive? #t)))))