dashboard/builder: pass GO386 to child builder, like GOARM

Update golang/go#7720

LGTM=dave
R=adg, dave
CC=golang-codereviews, rsc
https://golang.org/cl/84410046

dashboard/builder/env.go

@@ -20,6 +20,7 @@ import (
 // to the envv of its subprocesses.
 var extraEnv = []string{
 	"GOARM",
+	"GO386",
 	"CGO_ENABLED",
 
 	// For Unix derivatives.