Expose toolchain-preference as a CLI and configuration file option

[zanieb]

Exposes the option added in #4416. Adds --toolchain-preference and tool.uv.toolchain-preference to configure if system or managed toolchains are preferred. Users can opt-out of managed toolchains or system toolchains entirely as well.

[zanieb]

Should we bikeshed the name? Should it just be --toolchains?

[BurntSushi]

LGTM.

As for the name... Is this something that you think will be commonly used on the CLI? If so, I think I'd favor a shorter name. Otherwise, I like the descriptiveness of --toolchain-preference.

One other possible downside is redundancy here. Namely, --toolchain-preference prefer-system has "prefer" twice in it, but arguably is just as clear if "prefer" only appeared once. Not quite sure what the right answer is there.

[zanieb]

One other possible downside is redundancy here. Namely, --toolchain-preference prefer-system has "prefer" twice in it, but arguably is just as clear if "prefer" only appeared once.

Agree, this is a source of discomfort for me but idk it's also very clear.

I don't expect this to be used much from the CLI — mostly as a persistent configuration option.

[zanieb]

I'm fine adjusting this later if we need to since it's in preview.

[zanieb]

I guess another option is I drop the prefer prefix so we'd have

toolchain-preference = managed | system | only-managed | only-system | installed-managed

which is pretty reasonable too?

[BurntSushi]

I guess another option is I drop the prefer prefix so we'd have

toolchain-preference = managed | system | only-managed | only-system | installed-managed

which is pretty reasonable too?

I think that's probably okay. I thought of that too, but wondered whether it might leave folks wondering the difference between managed and only-managed.