Can not create python virtual environment because python3-venv package is not installed

Describe the bug

One difference with the GitHub ubuntu-latest runner (https://github.com/actions/virtual-environments/blob/2d46e3f4d965927ce242d1fed6d5e92c7ff076c5/images/linux/scripts/installers/python.sh#L17) is the presence of the python3-venv package.

This doesn't matter if you're just using the setup-python action for your own code, but it does when you're running another action (e.g. https://github.com/akaihola/darker/blob/master/action.yml) that tries to use a Python virtual environment to isolate itself from your code, then you hit the kind of brick wall explained in https://github.com/pypa/build/issues/224:

The virtual environment was not created successfully because ensurepip is not
available.  On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.

    apt install python3.8-venv

You may need to use sudo with that command.  After installing the python3-venv
package, recreate your virtual environment.

Failing command: ['/runner/_work/_actions/akaihola/darker/github-action-v1.3.2-1/.darker-env/bin/python3', '-Im', 'ensurepip', '--upgrade', '--default-pip']

Traceback (most recent call last):
  File "/runner/_work/_actions/akaihola/darker/github-action-v1.3.2-1/action/main.py", line 14, in <module>
    run([sys.executable, "-m", "venv", str(ENV_PATH)], check=True)
  File "/usr/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/usr/bin/python3', '-m', 'venv', '/runner/_work/_actions/akaihola/darker/github-action-v1.3.2-1/.darker-env']' returned non-zero exit status 1.

This is also reproducible by doing: docker run -it summerwind/actions-runner:latest python -m venv /tmp/boom or the same in a base Debian/Ubuntu setup.

Checks

• My actions-runner-controller version (v0.x.y) does support the feature
• I'm using an unreleased version of the controller I built from HEAD of the default branch

To Reproduce
Steps to reproduce the behavior:

Call python -m venv /tmp/path-to-venv inside a summerwind/actions-runner image.

Expected behavior

Python virtual env creation should work.

Screenshots
If applicable, add screenshots to help explain your problem.

Environment (please complete the following information):

• Controller Version [e.g. 0.20.2]
• Deployment Method [e.g. Helm and Kustomize ]
• Helm Chart Version [e.g. 0.11.0, if applicable]

Additional context
Add any other context about the problem here.

Start by inspecting the actions-runner image definition and reproduce python -m venv /tmp/path-to-venv in the summerwind/actions-runner image. Identify where its Python packages are installed; done means virtual environment creation succeeds without the ensurepip or python3-venv error.