gnu: emacs-go-mode: Fetch sources from git.

* gnu/packages/emacs.scm (emacs-go-mode)[source]: Fetch via git.
[arguments]: Add phase "make-writable".

gnu/packages/emacs.scm

@@ -1474,14 +1474,22 @@ current match, total matches and exit status.
     (name "emacs-go-mode")
     (version "1.5.0")
     (source (origin
-              (method url-fetch)
-              (uri (string-append "https://github.com/dominikh/go-mode.el/"
-                                  "archive/v" version ".tar.gz"))
-              (file-name (string-append name "-" version ".tar.gz"))
+              (method git-fetch)
+              (uri (git-reference
+                    (url "https://github.com/dominikh/go-mode.el.git")
+                    (commit (string-append "v" version))))
+              (file-name (git-file-name name version))
               (sha256
                (base32
-                "1adngbjyb8qnwg7n6r2y31djw9j6qf3b9fi63zd85035q7x4ljnm"))))
+                "1nd2h50yb0493wvf1h7fzplq45rmqn2w7kxpgnlxzhkvq99v8vzf"))))
     (build-system emacs-build-system)
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'make-writable
+           (lambda _
+             (for-each make-file-writable (find-files "." "\\.el$"))
+             #t)))))
     (home-page "https://github.com/dominikh/go-mode.el")
     (synopsis "Go mode for Emacs")
     (description