ci(workflows): remove parallel execution on PR runs

Author: harryswift01

.github/workflows/pr.yaml

@@ -111,7 +111,7 @@ jobs:
           python -m pytest tests/regression \
             -m "not slow" \
             -k "${{ matrix.system }}" \
-            -n auto \
+            -n 0 \
             --dist=loadscope \
             -vv \
             --durations=20