Prefer PR number for group instead of ref (#1147)

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### Changes

There are a few PRs that are stuck waiting for Browerstack tests to run,
when checking [that
action](https://github.com/auth0/auth0-spa-js/actions/runs/6250742960)
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.

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### Testing

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### Checklist

- [x] I have read the [Auth0 general contribution
guidelines](https://github.com/auth0/open-source-template/blob/master/GENERAL-CONTRIBUTING.md)
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.github/workflows/browserstack.yml

@@ -15,7 +15,7 @@ permissions:
   contents: read
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: ${{ github.ref != 'refs/heads/main' }}
 
 env: