I propose we add an choice option with the two values ['--override-update-channel', ''] this way if the just add the variable in the command line if is empty it won't do notting, and if is '--override-update-channel' will be boolean. 
https://github.com/cosmin-malutan/mozmill-ci/commit/81079100a81ce21f7e288ca1f11c3cfd200ec33b