gitlab ci: cache the whole gluon directory

Now that we've moved the make update to its own stage, it makes sense to cache
the whole gluon directory between targets because it won't be overwritten by
make update.

.gitlab-ci.yml

@@ -41,7 +41,8 @@ update:
   cache:
     paths:
       - gluon/openwrt/dl
-    key: "build"
+      - gluon/openwrt/staging_dir
+    key: "build-$CI_COMMIT_REF_SLUG"
  
 target:ar71xx-generic:
   extends: .target