ci: cancel previous runs when new pushes were made

This is more what is naturally expected. It saves CI time and it often
prevents many already started runs that will fail anyway from running
longer than necessary.

See https://stackoverflow.com/questions/66335225

.github/workflows/rust.yml

@@ -11,6 +11,10 @@ name: "Cargo workspace"
 # Run on every push (tag, branch) and pull_request
 on: [ pull_request, push, workflow_dispatch, merge_group ]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 env:
   CARGO_TERM_COLOR: always