diff --git a/.github/workflows/test-pull-request.yml b/.github/workflows/test-pull-request.yml
index 116b7ebc74..bc80115dfb 100644
--- a/.github/workflows/test-pull-request.yml
+++ b/.github/workflows/test-pull-request.yml
@@ -25,10 +25,13 @@ jobs:
           - windows-latest
         arch:
           - x64
+        prefix:
+          - ''
         include:
           - version: '1.6'  # test on oldest supported version
             arch: x64
             os: ubuntu-latest
+            prefix: 'JULIA_NUM_THREADS=1 '
     env:
       JULIA_PKG_SERVER: ''
     steps:
@@ -47,6 +50,8 @@ jobs:
         uses: julia-actions/julia-runtest@v1
         timeout-minutes: 30  # kill stale process earlier
         continue-on-error: ${{ matrix.version == '1.6' }}
+        with:
+          prefix: ${{ matrix.prefix }}
       - name: Process coverage
         uses: julia-actions/julia-processcoverage@v1
       - name: Upload coverage