I did not commit on purpose because this is actually to be found in the
official flathub repository for GIMP (the concept being to keep the
stable and nightly manifests as sync-ed as possible) and I thought there
is no need to duplicate data. But since this is apparently confusing
people (cf. !91), let's just push it.

Note that even though this patch is taken from the org.octave.Octave
flatpak package, it is actually slightly different (I tweaked it because
we needed to build even less options from SuiteSpace than they do
apparently).