Specify update channel for Krita Nightly AppImage build
requested to merge amedonosova/binary-factory-tooling:amedonosova/krita-appimage-channel-fix into master
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.