Comment 16 for bug 1754174

It appears the following one-liner patch fixes the permissions problem in Lubuntu, which is caused by /usr/share/ubiquity/plugininstall.py being called without elevated privileges.

=== modified file 'ubiquity/filteredcommand.py'
--- ubiquity/filteredcommand.py 2013-02-19 11:33:07 +0000
+++ ubiquity/filteredcommand.py 2018-04-05 22:14:08 +0000
@@ -193,6 +193,8 @@
                 # default action or else they won't notice if the
                 # debconffilter dies.
                 signal.signal(signal.SIGPIPE, signal.SIG_DFL)
+ # Regain root.
+ misc.regain_privileges()

             ret = subprocess.call(self.command, preexec_fn=subprocess_setup)
             if ret != 0:

It is not clear to me if this is correct, or why plugininstall.py is handled via this code path on Lubuntu when it evidently is not on other flavors (or alternatively: why this code is called with privileges already raised on all other flavors).