Fix use of sudo logic in Os.copy

[dustanddreams]

Os.copy is not used by anything, but just in case:

• the existing logic "if superuser then sudo copy else copy" obviously needs its condition inverted.
• but then Os.sudo can be used directly since it will only use sudo when not superuser.

[MisterDA]

It was needed at some point of the development of the Docker backend, and was probably going to be used for macOS (until it wasn't), which is why I included it earlier. The idea with superuser was to request the copy to be run as super user: it's not usually needed, but sometimes you want to copy files as root to a privileged destination. Note that OBuilder can run as a regular user, and uses sudo accordingly. I'm tempted to remove this function altogether since it's not used anymore, even in the Docker backed.

[dustanddreams]

The idea with superuser was to request the copy to be run as super user: it's not usually needed, but sometimes you want to copy files as root to a privileged destination.

Ok. Then I'll withdrawn this PR.