Add concurrency group for CI jobs

As it stands, an arbitrary number of CI jobs can be running for a given
pull request, which means that sometimes the CI will be running for a
commit that has been superceded by several newer commits. Adding a
concurrency group to the CI jobs allows GitHub to automatically cancel
old jobs if a new commit is pushed that makes the run obsolete.

.github/workflows/builds.yml

@@ -6,6 +6,11 @@ on:
     branches:
       - main
 
+# Cancel existing jobs on new pushes.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 # All the different build/test jobs.
 jobs:
 

.github/workflows/checks.yml

@@ -1,5 +1,10 @@
 name: Checks
 
+# Cancel existing jobs on new pushes.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 on:
   push:
   pull_request: