main: Only verify SUPERUSER flag if using default sysroot

main: Only verify SUPERUSER flag if using default sysroot

The use case for non-default sysroots that I know of are:
 1) The current test suite
 2) Installers (Anaconda)
 3) Inspecting VM disks

For 2) and 3), it'll quickly be obvious if they're not running as
root, and these are more obscure cases.  We want to allow 1), and this
is a simple way to do it.

https://bugzilla.gnome.org/show_bug.cgi?id=747164