build-manifest: stop generating numbered channel names except for stable
build-manifest: stop generating numbered channel names except for stable

This fixes numbered channel names being created for the nightly channel,
and once the root cause of this rides the trains, for beta.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust