Use --additional-flags='check-untyped-defs' when running mypy_primer (#9433)

Author: AlexWaygood

.github/workflows/mypy_primer.yml

@@ -52,6 +52,7 @@ jobs:
             --new v${MYPY_VERSION} --old v${MYPY_VERSION} \
             --custom-typeshed-repo typeshed_to_test \
             --new-typeshed $GITHUB_SHA --old-typeshed upstream_main \
+            --additional-flags="--check-untyped-defs" \
             --num-shards 4 --shard-index ${{ matrix.shard-index }} \
             --debug \
             --output concise \