ci: do not run CI multiple times on the same branch

Especially for PRs, if the branch is updated
multiple times, only the last commit should be tested.

.github/workflows/ci.yml

@@ -1,5 +1,11 @@
 name: Rust CI
 
+# Cancel previously started workflow runs
+# when the branch is updated.
+concurrency:
+  group: ${{ github.ref }}
+  cancel-in-progress: true
+
 on:
   pull_request:
   push: