gitlab: Don't build any gitlab component outside linux

For some reason hydra seems to have issues downloading the
gitlab-workhorse source on macOS. Since we don't build the rails app
for macOS, the other components seem a bit useless there, so we
limit them to linux for now.

pkgs/applications/version-management/gitlab/gitlab-shell/default.nix

@@ -25,7 +25,7 @@ buildGoPackage rec {
   meta = with stdenv.lib; {
     description = "SSH access and repository management app for GitLab";
     homepage = http://www.gitlab.com/;
-    platforms = platforms.unix;
+    platforms = platforms.linux;
     maintainers = with maintainers; [ fpletz globin talyz ];
     license = licenses.mit;
   };