TST: avoid unneeded venv activation in tox.yml

[neutrinoceros]

follow up to #353

I think this can be simplified, but let's see how CI actually react.

[Cadair]

Does this actually prevent a venv being made / used?

https://docs.astral.sh/uv/reference/cli/#uv-tool-install says:

Packages are installed into an isolated virtual environment in the uv tools directory.

[neutrinoceros]

it doesn't, but that's also not my goal. I just want to prevent the env to be activated explicitly, since working with uv allows us to not manage (de)activation manually (hence why activate-environment is false by default).
Anyway I missed that toxdeps would mess this up. It could be parsed so that each dep is prepended with a -w (--with) flag, but then the end result wouldn't be simpler than what we started with, so overall it doesn't seem worth it.