options for Develop() are not automatically added to Install(), so the
new option in commit 8fb6d5f37e8055ce720bd0b1d56587f88c0071f285966ba17e72b2b12672aa73
only worked for the former. And since consolidate_paths unconditionally
checked opts.system_plugins_location, trying to run an ordinary install
ended in:
File "/build/calibre-git/src/calibre/setup/install.py", line 117, in consolidate_paths
self.system_plugins_loc = opts.system_plugins_location
AttributeError: 'Values' object has no attribute 'system_plugins_location'