update nixpkgs

nix/sources.json

@@ -35,10 +35,10 @@
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "b313502c719069cce2dd6fd1d5e7fc5999b21c70",
-        "sha256": "0vqqdq7pmz8rkylf2wp5rrzi7qknihavscrkk2fcd8bnb81bcpiq",
+        "rev": "6c0c30146347188ce908838fd2b50c1b7db47c0c",
+        "sha256": "05bh6kw7jzfxpvhndsg6swgbk1x7fsgnwahzm3jgd61rsvknhn93",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/b313502c719069cce2dd6fd1d5e7fc5999b21c70.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/6c0c30146347188ce908838fd2b50c1b7db47c0c.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "simple-nixos-mailserver": {