gitlab-ci, build/linux: Give .flatpak artifact a "standard" name

Every artifact from our CI have a pretty name with some reason for it:
- Installer: it is this way on download.gimp.org since ever(?) and that's fine
- Store: follows Microsoft de facto spec of .msix files naming
- AppImage: follows AppImage draft spec of .appimage files naming

But the .flatpak just looks like AUR pkg. So, let's use reverse DNS
naming which is used at least for .flatpakref files. This makes more sense.

.gitlab-ci.yml

@@ -772,7 +772,7 @@ dist-flatpak-weekly:
   artifacts:
     expose_as: 'Linux flatpak'
     paths:
-      - gimp-git.flatpak
+      - org.gimp.GIMP.flatpak
     expire_in: 8 days
 
 dist-installer-weekly:

build/linux/flatpak/3_dist-gimp-flatpak.sh

@@ -13,7 +13,7 @@ fi
 # Generate a Flatpak "bundle" to be tested with GNOME runtime installed
 # (it is NOT a real/full bundle, deps from GNOME runtime are not bundled)
 echo '(INFO): packaging repo as .flatpak'
-flatpak build-bundle repo gimp-git.flatpak --runtime-repo=https://nightly.gnome.org/gnome-nightly.flatpakrepo org.gimp.GIMP ${BRANCH}
+flatpak build-bundle repo org.gimp.GIMP.flatpak --runtime-repo=https://nightly.gnome.org/gnome-nightly.flatpakrepo org.gimp.GIMP ${BRANCH}
 
 
 # Publish GIMP repo in GNOME nightly

devel-docs/gitlab-mr.md

@@ -26,8 +26,8 @@ the pipeline. Once the pipeline ends, the flatpak can be installed by:
 - clicking the pipeline ID.
 - In the "gimp" stage, click the "gimp-flatpak-x64" job.
 - Then click the "Browse" button.
-- Click the `gimp-git.flatpak` file to download it.
-- Locally run: `flatpak install --user ./gimp-git.flatpak`
+- Click the `org.gimp.GIMP.flatpak` file to download it.
+- Locally run: `flatpak install --user ./org.gimp.GIMP.flatpak`
   It should propose you to install the flatpak, allowing you to test.
 - After testing, you can uninstall with: `flatpak remove org.gimp.GIMP//master`