build: no required environment variables

R=adg, r, PeterGo
CC=golang-dev
https://golang.org/cl/1942044
diff --git a/src/cmd/goinstall/main.go b/src/cmd/goinstall/main.go
index 5d2af2e..ffa6eb7 100644
--- a/src/cmd/goinstall/main.go
+++ b/src/cmd/goinstall/main.go
@@ -14,6 +14,7 @@
 	"io"
 	"os"
 	"path"
+	"runtime"
 	"strings"
 )
 
@@ -28,7 +29,7 @@
 	errors  = false
 	gobin   = os.Getenv("GOBIN")
 	parents = make(map[string]string)
-	root    = os.Getenv("GOROOT")
+	root    = runtime.GOROOT()
 	visit   = make(map[string]status)
 
 	reportToDashboard = flag.Bool("dashboard", true, "report public packages at "+dashboardURL)