ci: Allow concurrency outside of pull requests

When multiple merges to `master` happen before a CI run completes, the
in-progress job is getting canceled. Fix this by using the commit sha
for the group key if a pull request number is not available, rather than
`github.ref` (which is always `refs/head/master` after merge). This
should prevent jobs running on previous commits from getting cancelled,
while still ensuring there is only ever one active run per pull request.

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