allow setting MISE_RUBY_INSTALL and other ruby configs in toml file

[glyh]

For now I have to put this in my fish config, which is not ideal.

[glyh]

BTW it would also be nice if user can set mirrors for the cache site.

[jdx]

you can use universal variables in fish

[glyh]

Yes and that's how it work on my machine. What if someday I decided to switch to another shell?