CI: Expire PR artifacts sooner and cancel superseded CI runs #615

@hannesa2, Two small things in BuildPR.yml - both your call since it also builds releases.

No retention-days on the four upload-artifact steps, so PR builds (~21 MB a run)
are kept the full 90 days, same as releases.
I suggest we keep these for just a week or two:

retention-days: ${{ github.event_name == 'pull_request' && 7 || 90 }}

No concurrency group - a (second) push to a PR-branch starts an extra full matrix-build
instead of replacing the running one.
We can set this to be keyed on the PR number, with a fallback so master and tag builds are never cancelled:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
  cancel-in-progress: true

Want me to open a PR for these?