build: set back snapcraft to a nightly build.

build/linux/snap/snapcraft.yaml

@@ -1,6 +1,6 @@
-name: gimp
+name: gimp-experimental
 adopt-info: gimp
-grade: stable #see parts:babl_or_gegl:override-pull
+grade: devel #see parts:babl_or_gegl:override-pull
 base: core24
 compression: lzo
 confinement: strict
@@ -171,7 +171,7 @@ parts:
       - -Dgi-docgen=disabled
       - -Dcheck-update=no
       - -Denable-default-bin=enabled
-      - -Dbuild-id=org.gimp.GIMP.snapcraft.latest
+      - -Dbuild-id=org.gimp.GIMP.snapcraft.experimental
     build-packages:
       - bison
       - flex