CI: Cancel redundant workflow runs

Each push operation to a feature branch would regularly trigger a CI
workflow run. When quickly pushing in succession, the previous runs
become redundant and obsolete instantly, so let's cancel them.

Author: amotl

.github/workflows/main.yml

@@ -7,6 +7,9 @@ on:
   release:
     types: [published]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   build: