CI: add concurrency support to zfs-arm

The zfs-arm workflow was the only build/test workflow without a
concurrency block, so superseded runs were not cancelled.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Christos Longros <chris.longros@gmail.com>
Closes #18608

.github/workflows/zfs-arm.yml

@@ -19,6 +19,10 @@ on:
         default: ""
         description: "(optional) install specific GCC version, like '16'"
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   zfs-arm:
     name: ZFS ARM build