cue/gen.go: fix bug in generation.

Change-Id: Ice79b78e07ee3d8a4a9090dd328bdeb808f61aa3
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6563
Reviewed-by: CUE cueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/builtins.go

@@ -124,6 +124,7 @@ func main() {
 	// TODO: do this in a more principled way. The best is probably to
 	// put all builtins in a separate package.
 	b = bytes.Replace(b, []byte("cue."), []byte(""), -1)
+	b = bytes.Replace(b, []byte("{{}}"), []byte("{}"), -1)
 
 	if err := ioutil.WriteFile("builtins.go", b, 0644); err != nil {
 		log.Fatal(err)