python/adbc_driver_manager: profiles don't work in virtualenvs

[lidavidm]

venvs don't work because we add the additional search path <venv root>/etc/adbc/drivers, and have no way to configure profiles.

Options are:

• Add the other search path; but then we search both paths for both manifests and profiles, and we risk confusing one for the other.
• We could additionally require profiles to have a different extension.
• Instead, we could add a second API function to set another search path. That's starting to get unwieldy.
• We could ditch the API function, and instead use normal driver options to set additional search paths for drivers/manifests.

Originally posted by @lidavidm in #4080 (comment)