Use bash to support all required commands

Shell on GitHub actions did not support the exec command. This enable the used of pre-commit in github actions.

With this a linting job can be configured as follows:

  lint:
    name: Linting
    runs-on: ubuntu-latest

    steps:
      - name: Checkout source
        uses: actions/checkout@v2

      - name: Setup Python
        uses: actions/setup-python@v2
        with:
          python-version: '3.8'

      - name: Update python environment
        run: |
          pip install --upgrade pre-commit

      - name: Run all linters
        run: pre-commit run --all-files