checkout: Add missing cancellable arg
checkout: Add missing cancellable arg

vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,GNOME/gnome-ostree,GNOME/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,chergert/gnome-continuous,GNOME/gnome-ostree,djdeath/gnome-ostree,chergert/gnome-continuous