curtin: desktop grub.replace_linux_default False

Grub provides "quiet splash" for GRUB_CMDLINE_LINUX_DEFAULT out of the
box, but curtin rewrites that value and drops both "quiet" and "splash".
On desktop, retain "quiet splash".

This should be configurable via autoinstall if this change is undesired.

subiquity/models/subiquity.py

@@ -517,6 +517,9 @@ def render(self):
             },
         }
 
+        if self.source.current.variant == "desktop":
+            config["grub"]["replace_linux_default"] = False
+
         if os.path.exists("/run/casper-md5check.json"):
             with open("/run/casper-md5check.json") as fp:
                 config["write_files"]["md5check"] = {