Allow testing the updater also with release and alpha channel

A while ago, we've done a couple of changes to make easier testing the updater.

Therefore, at the moment, setting var/override_updater_url in projects/firefox/config is enough to make that the new updater URL and to inject a custom public key to verify updates.

However, this works only for the nightly channel. That's great to check updater patches, but less great to test the update path between actual releases.

We should investigate and see if conditionally changing the release/alpha key is enough, or if we need to do more.