cmd/go: stop using $GOROOT and $GOBIN in script output
cmd/go: stop using $GOROOT and $GOBIN in script output

They were necessary to produce a canonical script
when we checked in build scripts, but now they're just
getting in the way.

Fixes issue 3279.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5796068

gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,gnanderson/go