Install librt in mypy_primer (#19925)

Fixes #19844

Author: ilevkivskyi

.github/workflows/mypy_primer.yml

@@ -67,6 +67,7 @@ jobs:
             --debug \
             --additional-flags="--debug-serialize" \
             --output concise \
+            --mypy-install-librt \
             | tee diff_${{ matrix.shard-index }}.txt
           ) || [ $? -eq 1 ]
       - if: ${{ matrix.shard-index == 0 }}