Adds basic support for customizing the Julia version by setting the JULIAUP_CHANNEL environmental variable. This support is limited in a couple ways:

• We don't check that the user is using juliaup. They could have installed julia some other way in which case the JULIAUP_CHANNEL does not do anything and setting the language_version will not have any effect. That however is the status quo (currently setting the language_version does not have any effect) and many users use juliaup, so I think we could just document that language_version support requires your julia to be from juliaup.
• If the user passes a channel which is not installed, they will get a juliaup error that the channel can not be found. If we were confident they were using juliaup we could install it on their behalf instead, but I'm not sure this is necessary.

split out from #3494