Support python-version-file argument for setup-python

[evansd]

This would allow us to avoid repeatedly hardcoding the Python version throughout the workflow files and give us a single place to update:
https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#using-the-python-version-file-input

I imagine using it like:

steps:
  - uses: "opensafely-core/setup-action@v1"
    with:
      python-version-file: ".python-version"
      cache-dependency-path: "requirements.*.txt"
      install-just: true