CI: cache flatpak-builder files

It is needed to make things faster.

.gitlab-ci.yml

@@ -110,6 +110,10 @@ packaging:
     - flatpak-builder --bundle-sources --repo=devel build ${MANIFEST_PATH}
     - flatpak build-bundle devel ${BUNDLE} --runtime-repo=${RUNTIME_REPO} ${APPID}
 
+  cache:
+    paths:
+    - .flatpak-builder/cache
+
   environment:
     name: review/$CI_COMMIT_REF_NAME
     url: https://gitlab.gnome.org/$CI_PROJECT_PATH/-/jobs/$CI_JOB_ID/artifacts/raw/${BUNDLE}