Remove --use-hwthread-cpus flag and trigger on pull request

[maxdinkel]

Description and Context

As queens is public now, we have more cores available on the test runner and can therefore remove the --use-hwthread-cpus flag for the mpi command.

The trigger for the ci is changed to pull_request.

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How Has This Been Tested?

Checklist

• My commit messages mention the appropriate issue numbers (advised but optional).
• I mention the appropriate issue numbers in this MR.
• I updated documentation where necessary.
• I updated the README where necessary
• I have added as (few,) small and fast tests as possible to cover my changes.

Additional Information

Interested Parties

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[sbrandstaeter]

Changes look good to me.