github/workflows: Cancel when branch is updated.

This adds a concurrency section to all github workflows to cancel any
in progress workflow when a branch is updated. This should cancel any
ongoing or queued workflows, e.g. when a pull request is updated.

Signed-off-by: David Lechner <david@pybricks.com>

.github/workflows/mpy_format.yml

@@ -9,6 +9,10 @@ on:
       - 'tests/**'
       - 'tools/**'
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     runs-on: ubuntu-20.04 # use 20.04 to get python2