ci: cancel previous jobs on ref update

Let's save the environment (and reduce the number of jobs in GH Actions
queues) by cancelling old jobs on a ref update (force push).

See: https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-for-github-actions#concurrency

.github/workflows/unit_tests.yml

@@ -11,6 +11,9 @@ on:
 jobs:
   build:
     runs-on: ubuntu-20.04
+    concurrency:
+      group: ${{ github.workflow }}-${{ matrix.run_phase }}-${{ github.ref }}
+      cancel-in-progress: true
     strategy:
       fail-fast: false
       matrix: