title: How can I use pip cache in github actions?
body: <p>I am having some issues with using cached pip in Github Actions. My workflow setup is</p>
  
  <pre><code>name: tests
  
  on: [push, pull_request]
  
  jobs:
    linux:
  
      runs-on: ubuntu-18.04
      strategy:
        max-parallel: 4
        matrix:
          python-version: [3.6, 3.7, 3.8]
  
      steps:
      - uses: actions/checkout@v1
      - name: Set up Python ${{ matrix.python-version }}
        uses: actions/setup-python@v1
        with:
          python-version: ${{ matrix.python-version }}
      - uses: actions/cache@v1
        with:
          path: ~/.cache/pip
          key: ${{ runner.os }}-pip-${{ hashFiles('**/requirements.txt') }}
          restore-keys: |
            ${{ runner.os }}-pip-
      - name: Install
        if: steps.cache.outputs.cache-hit != 'true'
        run: |
          pip install pytest pytest-cov bandit sphinx recommonmark
          python -m pip install --upgrade pip
          pip install .
      - name: Test with pytest
        run: |
          pytest --disable-pytest-warnings --cov-report=xml --cov=chepy --cov-config=.coveragerc tests/
          coverage report -m
      - name: Test with bandit
        run: |
          bandit --recursive chepy/ --ignore-nosec --skip B101,B413,B303,B310,B112,B304,B320,B410,B404
      - name: Test docs
        run: |
          make -C docs/ clean html
      - name: Upload to codecov
        uses: codecov/codecov-action@v1.0.3
        with:
          token: ${{secrets.CODECOV_TOKEN}}
          file: ./coverage.xml
  </code></pre>
  
  <p>The issue I am having is that in subsequent actions that are triggered, it will not using the cache, but instead installing from pip all over again. </p>
  
  <p>How can I make it use the cache?</p>