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 #3279.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5796068

src/cmd/go/build.go

@@ -929,8 +929,6 @@ func (b *builder) fmtcmd(dir string, format string, args ...interface{}) string
 	if b.work != "" {
 		cmd = strings.Replace(cmd, b.work, "$WORK", -1)
 	}
-	cmd = strings.Replace(cmd, gobin, "$GOBIN", -1)
-	cmd = strings.Replace(cmd, goroot, "$GOROOT", -1)
 	return cmd
 }