env: detect OmniOS correctly even if we're running different illumos bits

[richlowe]

My original code here assuming /etc/verions/build.illumos-omnios would exist on OmniOS, except it doesn't if you're running other illumos bits. Pull it out of /etc/os-release too to check.

This should stop us supressing packaging dependencies on onu'd OmniOS, and then failing to run tests (etc) because sudo isn't installed.