ci: Skip source_check stage when appropriate

We don't have to run the static analyzer on javascript sources when
no javascript source was changed.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/408

.gitlab-ci.yml

@@ -23,6 +23,9 @@ js_check:
     script:
         - find js -name '*.js' -exec js60 -c -s '{}' ';' 2>&1 | tee $JS_LOG
         - (! grep -q . $JS_LOG)
+    only:
+        changes:
+            - js/**/*
     artifacts:
         paths:
             - ${JS_LOG}