CI: restirct EC2 benchmarking to one instance

Signed-off-by: Matthias J. Kannwischer <[email protected]>

.github/workflows/bench_ec2_reusable.yml

@@ -70,6 +70,8 @@ jobs:
       contents: 'read'
       id-token: 'write'
     runs-on: ubuntu-latest
+    concurrency:
+      group: ec2-bench
     if: ${{ always() }} # The point is to make this step non-cancellable,
                         # avoiding race conditions where an instance is started,
                         # but isn't yet done registering as a runner and reporting back.
@@ -114,6 +116,8 @@ jobs:
   bench_nix:
     name: Bench (nix)
     runs-on: ${{ needs.start-ec2-runner.outputs.label }}
+    concurrency:
+      group: ec2-bench
     needs: start-ec2-runner # required to start the main job when the runner is ready
     if: ${{ inputs.compiler == '' }}
     steps:
@@ -166,6 +170,8 @@ jobs:
       - bench_nix # required to wait when the main job is done
       - bench_custom # required to wait when the main job is done
     runs-on: ubuntu-latest
+    concurrency:
+      group: ec2-bench
     if: ${{ always() }} # required to stop the runner even if the error happened in the previous jobs
     steps:
       - name: Configure AWS credentials