Pass --unshare-net to bwrap when building with --external-sources

Ensures that when `--external-sources` is used, the build does not require a working internet connection.

`--unshare-net` is always passed for the sysa phase.

lib/utils.py

@@ -14,7 +14,7 @@ import sys
 
 def run(*args, **kwargs):
     """A small wrapper around subprocess.run"""
-    arguments = [str(arg) for arg in args]
+    arguments = [str(arg) for arg in args if arg is not None]
 
     if kwargs.pop('verbose', False):
         print(arguments)