Allow only one GithubActions CI at the time

This is to make sure that e.g. deploy actions running at different
times concurrently push wrong versions or older commit overrides
newest one.

.github/workflows/ci.yaml

@@ -18,6 +18,8 @@ on:
         required: true
         type: boolean
         default: false
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
 jobs:
   build-and-test:
     runs-on: ubuntu-latest