Update nixos-unstable

channels/nixos-unstable/channel.json

 {
   "ref": "refs/heads/nixos-unstable",
   "url": "https://github.com/NixOS/nixpkgs-channels/",
-  "rev": "135073a87b7e2c631739f4ffa016e1859b1a425e",
-  "date": "2020-05-29T08:08:31+02:00",
-  "sha256": "0s5kgyi7764r4zm51zy4isyc4zgn4fajwqwnrgm7s1xbs6zickv5",
+  "rev": "55668eb671b915b49bcaaeec4518cc49d8de0a99",
+  "date": "2020-07-01T20:07:51+02:00",
+  "path": "/nix/store/83nzhd71x8z2z7ic1zvq8p84i3n10xqm-nixpkgs-channels",
+  "sha256": "0b2mrrjsdpxpyvnv98dph5av3xjps1mbd87x8510mnc4pfa2zc8z",
   "fetchSubmodules": false,
   "deepClone": false,
   "leaveDotGit": false