Fix cache location across OSes

.github/workflows/ci.yml

@@ -14,9 +14,13 @@ jobs:
     - uses: actions/setup-python@v1
       with:
         python-version: ${{ matrix.python-version }}
+    # https://github.com/actions/cache/blob/master/examples.md#using-a-script-to-get-cache-location
+    - id: pip-cache
+      run: |
+        python -c "from pip._internal.locations import USER_CACHE_DIR; print('::set-output name=dir::' + USER_CACHE_DIR)"
     - uses: actions/cache@v1
       with:
-        path: ~/.cache/pip
+        path: ${{ steps.pip-cache.outputs.dir }}
         key: ${{ runner.os }}-pip-${{ hashFiles('**/setup.py') }}
         restore-keys: |
           ${{ runner.os }}-pip-