Only use unshare to become root if we're actually going to use a scope

Only use unshare to become root if we're actually going to use a scope

If the relevant environment variables are not set, scope_cmd() will
return an empty list and we won't use a scope after all. In that case
we don't need to use unshare either to become root and can rely on our
own become_root() function so check whether we're actually going to use
a scope or not.