Add --prefer-offline / MISE_PREFER_OFFLINE

[hverlin]

Add a --prefer-offline flag / MISE_PREFER_OFFLINE environment variable for a few commands like mise doctor. I imagine this can be useful in various context when the internet speed is low or when one wants a command to return as fast as possible.

This is obviously not going to work with every command (like mise install) however.

[syhol]

Sounds good to me. I like how mise uses network by default but its nice to have an offline mode.