Don't install packages from external archive in prepare page

https://bugs.launchpad.net/somerville/+bug/1993367

It tries to install Nvidia driver required package during
verifying installation configuration. The procedure will be stuck in Dell
FI environment. Set it to false to omit the ubuntu-drivers install
on DGPU config.

ubiquity/dell-bootstrap.py

@@ -893,7 +893,7 @@ def prepare(self, unfiltered=False):
             self.preseed("ubiquity/minimal_install", "true")
 
         self.preseed_bool("ubiquity/download_updates", False)
-        self.preseed_bool("ubiquity/use_nonfree", True)
+        self.preseed_bool("ubiquity/use_nonfree", False)
 
         return (['/usr/share/ubiquity/dell-bootstrap'], [RECOVERY_TYPE_QUESTION])