Bump to nixpkgs with morph 1.3.1 instead of 1.3.0.dev

nixpkgs.json

 { "name": "nixpkgs"
-, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/b8240ab9525aaab8f8587e8aaf7be99fcca5c220.tar.gz"
-, "sha256": "1ilq964f1qv0ajf8abfqz88q8p8s5lzbxjlfm8969krpp2i5sgp7"
+, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/c54e4650cadc90152a345e070d7036cdda3aae8e.tar.gz"
+, "sha256": "1mzqqzqdzkqn16j6kd7zw7v1v0symjg7arw605vksx4n1ydpbx6k"
 }