Specify update channel for Krita Nightly AppImage build

The AppImage build scripts try to automatically detect the right update
channel for nightlies from current git branch. Due to the way the source
is handled on the Binary Factory, this detection does not work (the
source is in the detached head state).

To work around that, this commit adds an argument to build-krita.sh and
build-image.sh scripts, which overrides autodetection of update channel.