Cancel running workflows automatically on PR update (#31044)

* Cancel running workflows automatically on PR update
* Add the last update later to check cancellation is working
* Use github.run_number instead of github.sha

.github/workflows/unit_tests.yaml

@@ -9,6 +9,11 @@ on:
     branches:
       - develop
       - releases/**
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_number }}
+  cancel-in-progress: true
+
 jobs:
   # Validate that the code can be run on all the Python versions
   # supported by Spack