nightly: fix check for Illumos libc

nightly: fix check for Illumos libc

On Illumos `uname -o` outputs "illumos". We weren't testing for
that so it fell through to the Linux block which runs libc.so to
get the version string. On Illumos that outputs

usage: ld.so.1 [-e option,...] dynamic-object [object args,...]