Merge pull request #586 from liyier90/cicd-enable-pip-cache

cicd: enable pip cache in pre merge CI

.github/workflows/pre_merge_ci.yml

@@ -14,6 +14,7 @@ jobs:
         uses: actions/setup-python@v2
         with:
           python-version: 3.7
+          cache: "pip"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip