cache pip directory on github actions (#5365)

.github/workflows/tests.yaml

@@ -52,7 +52,20 @@ jobs:
       uses: actions/[email protected]
       with:
         python-version: ${{ matrix.pyv }}
+    - name: get pip cache dir
+      id: pip-cache-dir
+      run: |
+        echo "::set-output name=dir::$(pip cache dir)"
+    - name: set pip cache
+      id: pip-cache
+      uses: actions/cache@v2
+      with:
+        path: ${{ steps.pip-cache-dir.outputs.dir }}
+        key: ${{ runner.os }}-pip-${{ hashFiles('setup.py') }}
+        restore-keys: |
+          ${{ runner.os }}-pip-
     - name: install
+      if: steps.cache.pip-cache-dir.cache-hit != 'true'
       run: |
         pip install wheel
         pip install -e ".[all,tests]"