skip to set back-ref if it is immutable

[lesomnus]

fixes https://github.com/ent/ent/issues/3843

[lesomnus]

~~Can I use latest version of protoc-gen-go-grpc and protoc to test?~~

 // versions:
-// - protoc-gen-go-grpc v1.2.0
-// - protoc             v3.19.4
+// - protoc-gen-go-grpc v1.3.0
+// - protoc             v4.24.0
 // source: entpb/entpb.proto

Never mind. I downgraded version of mine.