Project Management: Fix pull request merge automation errors (#19768)

* Framework: Use fixed version of checkout action

Avoid unintended breaking changes. To a lesser extent, helps clarify that this tag refers to the _version of the action_, not the branch being checked out.

* Framework: Configure PR automation checkout to master branch

.github/workflows/pull-request-automation.yml

@@ -7,7 +7,11 @@ jobs:
   pull-request-automation:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@master
+      # Checkout defaults to using the branch which triggered the event, which
+      # isn't necessarily `master` (e.g. in the case of a merge).
+      - uses: actions/checkout@v2
+        with:
+          ref: master
       # Changing into the action's directory and running `npm install` is much
       # faster than a full project-wide `npm ci`.
       - run: cd packages/project-management-automation && npm install