Prefer PR number for group instead of ref

[ewanharris]

Changes

There are a few PRs that are stuck waiting for Browerstack tests to run, when checking that action we can see the following Browserstack: .github#L1 Canceling since a higher priority waiting request for 'Browserstack-refs/heads/main' exists, and this group key is consistent across all 3 PRs.

I believe this is because we use pull_request_target where the ref is the base branch (branch the PR is targeting), by preferring the PR number where possible we ensure it's unique for PRs.

Usual caveat applies of not being able to thoroughly test until merged, but based on other repos across GitHub I think usage of nested objects that may not exist is safe.

References

Testing

• This change adds unit test coverage
• This change adds integration test coverage
• This change has been tested on the latest version of the platform/language

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All code quality tools/guidelines have been run/followed