all: add env support, freebsd-386, freebsd-amd64-race, re-enable plan9

Update golang/go#9491
Update golang/go#8639

Change-Id: I1f43c751777f10a6d5870ca9bbb8c2a3430189bf
Reviewed-on: https://go-review.googlesource.com/3170
Reviewed-by: Andrew Gerrand <adg@golang.org>
6 files changed