CI: cancel older concurrent PR runs

- Remove the space in the workflow name to ensure there are no side
  effects when using it as an identifier.
- Act on PRs only, not on upstreaming.

Signed-off-by: Axel Heider <axelheider@gmx.de>

.github/workflows/sel4test-hw.yml

@@ -6,7 +6,7 @@
 #
 # See sel4test-hw/builds.yml in the repo seL4/ci-actions for configs.
 
-name: seL4Test HW
+name: seL4Test-HW
 
 on:
   # needs PR target for secrets access; guard by requiring label
@@ -17,6 +17,11 @@ on:
 permissions:
   contents: read
 
+# Cancel older runs to reduce the load, especially on the machine queue.
+concurrency:
+  group: ${{ github.workflow }}-pr-${{ github.event.number }}
+  cancel-in-progress: true
+
 jobs:
   hw-build:
     name: HW Build

.github/workflows/sel4test-sim.yml

@@ -6,13 +6,20 @@
 #
 # See sel4test-sim/builds.yml in the repo seL4/ci-actions for configs.
 
-name: seL4Test Sim
+name: seL4Test-Sim
 
 on:
   push:
     branches: [master]
   pull_request:
 
+# Cancel older runs to reduce the load. The workflow also runs on pushes to
+# the master brnach, but skipping or cancellation will not happen there
+# practically due to the unique run ID.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && format('pr-{0}', github.event.number) || format('run-{0}', github.run_id) }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   simulation:
     name: Simulation