rkt: build a limited version on non-Linux systems

From https://github.com/coreos/rocket/issues/158#issuecomment-65279747, @philips said:
> Right now `rkt` is only designed to work on Linux hosts. Since `rkt` directly executes the binaries and doesn't talk to an API having `rkt` on Darwin doesn't have a lot of utility at this point. I will update the build scripts to fail on darwin with a more useful message.
I thought it was import order but I forget based on the spec.
On Dec 5, 2014 12:40 PM, "Jonathan Boulle" <notifications@github.com> wrote:

> In rkt/run_other.go:
>
> > @@ -0,0 +1,18 @@
> > +//+build !linux
> > +
> > +package main
> > +
> > +import "fmt"
> > +
> > +var (
> > +	cmdRun = &Command{
>
> Fair enough. Although then the order is non-deterministic, right?
>
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