ci: use git submodules for (securely) using third party Github Actions (#12709)

* Use git submodules for (securely) using third party Github Actions

List of repositories added as submodules:

EndBug/latest-tag@latest
morrisoncole/pr-lint-action@v1.4.1
trilom/file-changes-action@v1.2.4
styfle/cancel-workflow-action@0.6.0
apache-superset/cached-dependencies@b90713b
unsplash/comment-on-pr@v1.2.0

.github/workflows/docker.yml

@@ -13,8 +13,10 @@ jobs:
     name: build
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout
+      - name: "Checkout ${{ github.ref }} ( ${{ github.sha }} )"
         uses: actions/checkout@v2
+        with:
+          persist-credentials: false
 
       - shell: bash
         env: