Don't implicily enable --unshare-user when not running as root

[DaanDeMeyer]

If already in a user namespace, a regular user can have all the required permissions necessary for bubblewrap to function correctly. Hence bubblewrap shouldn't implicitly enable --unshare-user when not running as root if the user has all the required capabilities for bubblewrap to function correctly.

[smcv]

I'm guessing you have a use-case in mind for this. What is that use-case?

Please could you put together a merge request for the behaviour you would prefer to see?