build: don't build goplay in run.rc. Fix plan 9 build.
build: don't build goplay in run.rc.
Fix plan 9 build.

TBR=rsc
R=golang-codereviews
CC=golang-codereviews
https://codereview.appspot.com/100880047

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang