build/linux: Use custom App ID on Nightly flatpak
Using custom app ids is a common pratice in GNOME nightly apps and should workaround the fact the DEs are pretty dumb and confuse different GIMPs if they don't have different APP IDs.
React with org.gimp.GIMP.Devel
React with org.gimp.GIMP.Nightly
React with org.gimp.GIMP.Git
Edited by Bruno