ci: Also store gvc subproject in artifacts

After the subproject moved to a meson wrap, its checkout is no
longer managed by gitlab.

While meson will take care of checking out required submodules
during setup, this will not happen for `test` or `dist`, so store
the checked out code in artifacts.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3370>

.gitlab-ci.yml

@@ -203,6 +203,7 @@ build:
     paths:
       - mutter
       - build
+      - subprojects/gvc
       - subprojects/jasmine-gjs
 
 test: