run.bash: fix linux/arm build

Revert to the shell builtin to avoid hosts that do not have /usr/bin/time.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/6848054

src/run.bash

@@ -115,7 +115,8 @@ echo '#' ../test/bench/go1
 go test ../test/bench/go1
 
 (xcd ../test
-GOMAXPROCS= time go run run.go
+unset GOMAXPROCS
+time go run run.go
 ) || exit $?
 
 echo