Cancel outdated CI runs

Adds a concurrency group to the CI workflows, so a new push to a pull
request or branch cancels the runs still going for the previous commit
instead of letting them finish for nothing. The group is
github.workflow_ref (workflow file plus ref, refs/pull/<number>/merge
for pull requests), so two workflows with the same name and two fork
pull requests with the same branch name never cancel each other.
Release, notify and docs deploy workflows are left as they are.

Author: DavidBadura

.github/workflows/ci.yaml

@@ -19,6 +19,10 @@ permissions:
   contents: read
   packages: write
 
+concurrency:
+  group: ${{ github.workflow_ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Build per-arch (push by digest on main)