ci(workflows): update .github/weekly-regression.yaml to run in serial

Author: harryswift01

.github/workflows/weekly-regression.yaml

@@ -82,7 +82,7 @@ jobs:
           pytest tests/regression \
             -k "${{ matrix.system }}" \
             --run-slow \
-            -n auto \
+            -n 1 \
             --dist=loadscope \
             -vv \
             --durations=20