desktop: fix AppData unit testing when in RC+git code.

We still want the normal GIMP_RELEASE C macro to be set because the
whole development frame should have code behaving like release code, yet
only for the AppStream metadata testing, we want to allow a "TODO"
release date while in +git code.

desktop/meson.build

@@ -49,7 +49,7 @@ if appstreamcli.found()
     find_program('test-appdata.sh', required: true, dirs: [meson.current_source_dir()]),
     env: [
       'GIMP_TESTING_BUILDDIR=' + meson.current_build_dir(),
-      'GIMP_RELEASE=' + (release ? '1' : '0'),
+      'GIMP_RELEASE=' + (release and not gimp_rc_git ? '1' : '0'),
     ],
     suite: 'desktop',
   )