(Some containers are deprivileged and cannot even run setpriority.
Traditional nice is helpful to linktest, because even if its setpriority
call fails, it still runs the command.  busybox nice just fails (which
is arguably the right behavior, but it's not consistent).  So now we do
a quick trial of the platform nice to see what it can do, and only use
it if it succeeds on /bin/true.)