Make the default channel configurable via an environment variable

[lassepe]

I would like to migrate from asdf-vm over to juliaup. The only missing feature for me to do that would be to be able to set a channel per directory. I don't think that the feature of setting a version per directory should be handled by juliaup itself because it seems out of scope. Instead, I think it would be nice to use something like direnv to export an environment variable like JULIAUP_CHANNEL. Upon start of the julia launcher, the launcher can check if this environment variable is set and use it as a default channel if the user does not request a specific channel via julia +channel.

[davidanthoff]

Yes, entirely agreed. I think we also want something file and Julia specific, see https://github.com/JuliaLang/juliaup/issues/10. At the end we'll need something like the rustup hierarchy of configuration locations for this. But one should definitely be an env variable, and then that should make it all work easily with direnv for those that want to go that route.