build: Disable grass submodules

It seems that they are only necessary for some tests, that we don't run.

build-aux/org.gnome.Fractal.Devel.json

@@ -68,7 +68,8 @@
                     "type": "git",
                     "url": "https://github.com/connorskees/grass",
                     "tag": "0.13.4",
-                    "commit": "e0bb9e2eabfc3a58e42b03089cd7b22c68d09d0b"
+                    "commit": "e0bb9e2eabfc3a58e42b03089cd7b22c68d09d0b",
+                    "disable-submodules": true
                 }
             ]
         },