build/linux: Fix flatpak ignoring fork source code

See: https://gitlab.gnome.org/GNOME/gimp/-/merge_requests/1670#note_2152565

This makes the NIGHTLY flatpak use the local GIMP source. This reduces the
internet usage and makes possible to generate .flatpak from every branch out
there, which turns the flatpak finally really useful to code development.

Of course, this makes impossible to run 'flatpak-builder NIGHTLY_manifest.json'
without cloning or downloading GIMP code, but this is not a regression since:

1) the NIGHTLY manifest is in-source to begin with, so same thing as our other
   building methods (except macOS), which runs over the code they are stored

2) the chances of someone downloading partial source code (the NIGHTLY manifest
   alone) so being unable to build are low since this file isn't too easy to find

3) even if someone do this and don't like, we are not violating flatpak building
   pratices since this option exists to be used (and will be in NIGHTLY only)

build/linux/flatpak/org.gimp.GIMP-nightly.json

@@ -828,9 +828,8 @@
             ],
             "sources": [
                 {
-                    "type": "git",
-                    "url": "https://gitlab.gnome.org/GNOME/gimp.git",
-                    "branch": "master"
+                    "type": "dir",
+                    "path": "../../.."
                 },
                 {
                     "type": "shell",