Bump to nixpkgs with morph 1.4.0

morph 1.5.0 fails to build with the golang runtime surrounding it

nixpkgs.json

 { "name": "nixpkgs"
-, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/9a428b2a925f8fe01af4d81b28507c23e375dc18.tar.gz"
-, "sha256": "1glm4znrjn1jww55nls4b131nv6qgd3j9lnk1b9n2rql508an1db"
+, "url": "https://github.com/PrivateStorageio/nixpkgs/archive/d7c46f46c3e6acfcde44bb05877fd5c9f07dfe1b.tar.gz"
+, "sha256": "1vddxhp3ljaq9qqzbpjp2kx34ps7nzbsn4zy31x0748a52rlsh0q"
 }