ci: cancel in-progress Test and Build runs on PR update

ci: cancel in-progress Test and Build runs on PR update

When a contributor pushes a new commit to an open PR, the previous
Test and Build run is no longer informative and only consumes a
runner slot that the new run could use. Add a concurrency group
keyed on the workflow name and ref so a fresh push cancels the
prior in-progress run for the same PR.

Since this workflow only triggers on pull_request, the ref is
always refs/pull/<num>/merge (unique per PR), so cancel-in-progress
can be set unconditionally.

Co-Authored-By: Claude Opus 4.7 (1M context) <redacted>
Signed-off-by: Hauke Mehrtens <redacted>