.github/workflows: Cancel redundant workflow runs

Evern since the move to github actions in 70daa61 (CI: add GH actions for
main and release builds, 2023-04-20) we just executed the workflows for
every branch push.

This can quickly result in the workflow queue being clogged. There is also
no point in executing older versions of the same branch when a newer
version is available.

The version here supports multiple workflows [1]. And also cancels, on
purpose, redundant workflows on main and the release branches. See [2]
for a future way to don't do that on special branches.

[1]: https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-only-cancel-in-progress-jobs-or-runs-for-the-current-workflow
[2]: https://stackoverflow.com/questions/68418857/how-to-cancel-existing-runs-when-a-new-push-happens-on-github-actions-but-only

.github/workflows/build-pr.yml

@@ -31,6 +31,9 @@ on:
 defaults:
   run:
     shell: bash
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
 
   BuildInstaller:

.github/workflows/build-release.yml

@@ -8,6 +8,9 @@ on:
 defaults:
   run:
     shell: bash
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 jobs:
 
   CallPR: