fix concurrency for main branch jobs

.github/workflows/run-tests.yml

@@ -42,7 +42,10 @@ on:
 # cancel older runs of a pull request;
 # this will not cancel anything for normal git pushes
 concurrency:
-  group: cancel-old-pr-runs-${{ github.workflow }}-${{ github.ref_name }}
+  # * For runs on other repos, always use the `ref_name` so each branch only can have one concurrent run
+  # * For runs on `com-lihaoyi/mill`, use `head_ref` to allow one concurrent run per PR, but `run_id` to
+  #   allow multiple concurrent runs in master
+  group: cancel-old-pr-runs-${{ github.workflow }}-${{ (github.repository != 'com-lihaoyi/mill' && github.ref_name) || (github.head_ref || github.run_id) }}
   cancel-in-progress: true
 
 jobs: