Add nightly release channel with auto-updates

[ammario]

Originally reported by @S1M0N38 on Discord:

What is the easy way to use a rolling release of Mux (not the stable one, but the one released as artifacts of CI/CD)? Perhaps having the repo locally, pulling the main branch, and compiling? Maybe having nightly/canary flags in settings could help the growing user test base avoid reporting issues that have already been fixed in the main branch but are still present in the stable release.

Proposal:

• Opt-in to nightly release in the auto-updater tooltip, perhaps just a checkbox
  • 
• Release a mux@nightly tag on NPM alongside