doc-clean fails with absolute venv paths

Hi docs team 👋

I've been trying to integrate the starter pack a little more with our CMake project. To make things a little cleaner, we've decided to keep all possible directories under our CMake build directory when invoking the docs Makefile from CMake. This includes the virtual env directory as well.

CMake resolves all of its paths to absolute paths. And the following check fails because it seems to ask the following questions:
1. Does the venv directory exist?
2. Is it a directory?
3. Is the given path _not_ an absolute path

https://github.com/canonical/sphinx-docs-starter-pack/blob/b4eb8de6b6f82ec815bafe92a38fcdf770fd1896/docs/Makefile#L94

Removing the last check seems to fix things. But it seems wierd in my opinion. Why assume that the virtual env path is relative?