Fixes for alias feature

[MilesCranmer]

Changes:

1. args are now correctly propagated. So you can do juliaup link n +nightly -- --startup-file=no and this will propagate (before the args were not passed through)
2. I reordered the match statements relevant to the PR to always be DirectDownloadChannel, SystemChannel, LinkedChannel, AliasChannel - which is the same as the enum (sadly there's no rustfmt option to automate this)
3. I integrated some of my other comments in Expand link to handle aliases. DRY tests mechanics. #1237