get concurrent OS builds

Author: sjpb

.github/workflows/fatimage.yml

@@ -11,10 +11,10 @@ name: Build fat image
 jobs:
   openstack:
     name: openstack-imagebuild
-    concurrency: ${{ github.ref }}-{{ matrix.os_version }} # to branch/PR + OS
     runs-on: ubuntu-20.04
     strategy:
       matrix:
+        concurrency: ${{ github.ref }}-{{ matrix.os_version }} # to branch/PR + OS
         os_version: [RL8, RL9]
         rl9_selected:
           - ${{ inputs.use_RL9 == true }} # only potentially true for workflow_dispatch