How can I use pip cache in github actions?

I am having some issues with using cached pip in Github Actions. My workflow setup is

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

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.

How can I make it use the cache?

You forgot about adding id in step which uses actions/cache. This is required for condition in Install step to work.

- uses: actions/cache@v1
  id:   cache
  with:
   ...