nix: bump to HEAD of 21.11

nix/sources.json

@@ -12,15 +12,15 @@
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
-        "branch": "nixpkgs-unstable",
+        "branch": "nixos-21.11",
         "description": "A read-only mirror of NixOS/nixpkgs tracking the released channels. Send issues and PRs to",
         "homepage": "https://github.com/NixOS/nixpkgs",
         "owner": "NixOS",
-        "repo": "nixpkgs-channels",
-        "rev": "10100a97c8964e82b30f180fda41ade8e6f69e41",
-        "sha256": "011f36kr3c1ria7rag7px26bh73d1b0xpqadd149bysf4hg17rln",
+        "repo": "nixpkgs",
+        "rev": "9ab7d12287ced0e1b4c03b61c781901f178d9d77",
+        "sha256": "0bbd2pgcyavqn5wgq0xp8p67lha0kv9iqnh49i9w5fb5g29q7i30",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs-channels/archive/10100a97c8964e82b30f180fda41ade8e6f69e41.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/9ab7d12287ced0e1b4c03b61c781901f178d9d77.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }