Allow jobs that run without a venv

[lordmauve]

Description

It would be useful to allow running commands that do not need a venv at all. This would allow skipping the venv build if the venv is not needed.

For example, system commands or tools from non-Python package ecosystems (npx run prettier) should run without a venv.

With no venv, it would by typical to use once = true, but there could be a use case for once = false and to substitute {python_version} into the run command.

Details

• OS: Linux
• Python version: 3.12
• thx version: 0.6.0
• Can you repro on main? N/A
• Can you repro in a clean virtualenv? N/A