Remove branch filtering from build

[Michael1993]

Proposed commit message:

Remove branch filter on PR triggers (#673 / #674)

This commit removes the branch filtering on PRs. It prevented the
build pipeline from triggering in certain cases but we want to
trigger it for all PRs, so the filtering just did not make sense.

Closes: #673
PR: #674

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PR checklist

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[aepfli]

Lgtm