Without this commit, PRs can fail to be merged via the merge queue, as
it'll fail with this error:
```
Run scripts/check-release-notes.sh
PR gh-readonly-queue didn't update release notes
Error: Process completed with exit code 1.
```
Without this commit, the build may break if a PR commit is created, as
that shows the `PR_NUMBER` field we parse out as "master", reflecting
that its a merge commit into the master branch.
In this commit, we add a simple bash script to parse out the current PR
number from an environment variable in the GH actions context, and use
that to check to see if the PR has been referenced in the release notes
or not. This isn't 100% fool proof, but it should catch most of the
common cases.
