Add concurrency: directive to CI workflow

This cancels any in-progress runs of this workflow when a new commit is
pushed to `master`.

This seems desirable because the integration tests can take a long time
to run.

.github/workflows/go.yml

@@ -5,6 +5,10 @@ on:
   pull_request:
     branches: [ master ]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 env:
   CMGR_LOGGING: debug