Add --prefer-offline / MISE_PREFER_OFFLINE

[jdx]

Discussed in #3928

^{Originally posted by hverlin January 3, 2025}
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.