Set concurrency key. (#1112)

As used for similar Foundation builds.

Author: microbit-matt-hillsdon

.github/workflows/build.yml

@@ -7,9 +7,9 @@ on:
     branches:
       - "**"
 
-# This is conservative: ideally we'd include branch and stage in this key
-# https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#concurrency
-concurrency: deploy-python-editor-v3
+concurrency:
+  group: ${{ github.workflow }}-${{ startsWith(github.ref, 'refs/tags/v') && 'release' || github.ref }}
+  cancel-in-progress: ${{ github.ref != 'refs/heads/main' && !startsWith(github.ref, 'refs/tags/v') }}
 
 jobs:
   build: