Think about changing update-channel name for nightly
Currently we set var/torbrowser_update_channel to default for the nightly builds.
I see that Mozilla is using nightly for their nightly builds.
Maybe we could change it to nightly too.
It is possible however that changing this channel name has some effects on the build. One of them is the filename of the file containing the mar signing key, in toolkit/mozapps/update/updater/moz.build. However from a quick look, I didn't see other important changes that would be caused by using nightly as MOZ_UPDATE_CHANNEL.
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