Introduce `install_command_post` executed with ` --notest`

[webknjaz]

So it's now quite popular to use tox --notest in CIs in a separate task/step to make it easier to distinguish between failing deps setup vs actual tests.

The concrete use-case I have in mind is running pre-commit install-hooks, that downloads hook repos and initializes virtualenvs for them, separately from pre-commit run.

pre-commit install-hooks should be executed with tox --notest while pre-commit run should go under commands.

This may or may not be a duplicate of #715, up to you.

[gaborbernat]

My available efforts at the moment are aimed at fixing this as part of https://github.com/tox-dev/tox/issues/1394, but that probably will take a while (ETA September). I think we can do it on top of that.