build: inline external Github Actions to unblock CI (#12241)

* build: inline cached-dependencies to unblock CI

* Run E2E on pull_request on;y

* Inline all external actions

* Checkout needed for internal actions

Also fixes pre-commit

* Add missing files

.github/actions/comment-on-pr/README.md

@@ -0,0 +1,27 @@
+# Comment on PR via GitHub Action
+
+A GitHub action to comment on the relevant open PR when a commit is pushed.
+
+## Usage
+
+- Requires the `GITHUB_TOKEN` secret.
+- Requires the comment's message in the `msg` parameter.
+- Supports `push` and `pull_request` event types.
+
+### Sample workflow
+
+```
+name: comment-on-pr example
+on: pull_request
+jobs:
+  example:
+    name: sample comment
+    runs-on: ubuntu-latest
+    steps:
+      - name: comment PR
+        uses: unsplash/comment-on-pr@master
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          msg: "Check out this message!"
+```