Release channels for the Tuist CLI

The Tuist CLI is now distributed through three release channels instead of every CLI change immediately becoming a stable tag. The recommended install (tuist@latest or tuist@4.200) moves slowly and only receives backported fixes, while canary (X.Y.0-canary.N) is cut on every CLI-touching commit to main and rc (X.Y.0-rc.N) is cut from a frozen releases/X.Y.x branch for soaking before promotion to stable. Patches on a stable line continue using the existing cli-backport.yml workflow. These channels are opt-in for canary and rc, with stable remaining the default.