Only use uv to generate lockfiles

[DavidCEllis]

In the current release ducktools-env will use uv to install packages and possibly Python runtimes. This PR moves away from that to using pip always even if uv is available.

This is partly a maintenance issue, it's easier to know the pip fallback works if the pip fallback is just the method. Given this is what would be used if the user did not have uv it makes more sense to keep this method, even if the install is slightly slower. If the environment is cached, neither pip nor uv are used so it only matters when new environments are constructed.

The only place where there isn't equivalent functionality from pip or the stdlib is for generating universal lockfiles. In this case uv is still required at present.

Functionally the main difference this makes is that ducktools-env will no longer install uv python runtimes automatically. It's debatable whether it was a good idea to do that in the first place.