Append to /org/gnome/desktop/input-sources/sources instead of prepending
Prepending to the list created unexpected behaviour and race conditions. Unexpected behaviour in that gnome-shell remembers its current source is the first in the list and changing that entry will suddenly change the current source.
But we can't just fix gnome-shell because there's also a race between the script running and gnome-shell starting (or restarting). If the script is run before gnome-shell starts then the only way to communicate to the shell that we don't want to change the default is to avoid modifying the head of the list.