parallel job limit for doctest (#30342)

limit

Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

.github/workflows/doctests.yml

@@ -52,6 +52,7 @@ jobs:
     name: "Call doctest jobs"
     needs: setup
     strategy:
+      max-parallel: 1  # 1 split at a time (in `doctest_job.yml`, we set `8` to run 8 jobs at the same time)
       fail-fast: false
       matrix:
         split_keys: ${{ fromJson(needs.setup.outputs.split_keys) }}