Limit concurrent Github Actions builds

.github/workflows/contributor-build.yml

@@ -13,6 +13,16 @@ on:
   pull_request:
     branches:
       - 'main'
+
+# See https://github.com/hibernate/hibernate-orm/pull/4615 for a description of the behavior we're getting.
+concurrency:
+  # Consider that two builds are in the same concurrency group (cannot run concurrently)
+  # if they use the same workflow and are about the same branch ("ref") or pull request.
+  group: "workflow = ${{ github.workflow }}, ref = ${{ github.event.ref }}, pr = ${{ github.event.pull_request.id }}"
+  # Cancel previous builds in the same concurrency group even if they are in process
+  # for pull requests or pushes to forks (not the upstream repository).
+  cancel-in-progress: ${{ github.event_name == 'pull_request' || github.repository != 'hibernate/hibernate-orm' }}
+
 jobs:
   build:
     name: Java 11