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) <noreply@anthropic.com>
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

.github/workflows/multi-arch-test-build.yml

@@ -7,6 +7,10 @@ permissions:
   contents: read
   pull-requests: write
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   formalities:
     name: Test Formalities