SetEnv: we only need to expand v once
SetEnv: we only need to expand v once

We only need to expand the passed-in value once, not once for every variable
that is set in the environment.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #1607
Approved by: TomSweeneyRedHat

TomSweeneyRedHat/buildah,rhatdan/buildah,rhatdan/buildah,projectatomic/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,projectatomic/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah,nalind/buildah-1,rhatdan/buildah,nalind/buildah