Allow specifying custom com.mojang dirs in Regolith commands

[Nusiq]

Currently we're using environment variables to find the export targets for Regolith on non-Windows operating systems. Windows users don't even have that because the paths are hardcoded.

This is a nice feature, especially if someone doesn't have a specific version of Minecraft. I like it but I don't think the environment variables are the best way to handle it. Instead of that I suggest adding these paths to regolith config.

On top of that I want to add a --config flag that lets you overwrite the config settings just for a single Regolith run.