Fix https://github.com/jpetazzo/nsenter/issues/21

Prefix the shell command with `sudo ` if the user is not `root`.
Assumes that `sudo` exists.
Basically meant; because this variable is used later on, would the script give something like an "undefined variable" error? Sorry, if this is a silly comment, shell scripting is not my best point, and not behind my computer right now to test it :smile: