switch from the nixos release tag to branch

the branch has some python packaging fixes in nixpkgs that we need

nix/sources.json

@@ -42,15 +42,15 @@
         "url_template": "https://releases.nixos.org/nixos/21.05/nixos-21.05.3740.ce7a1190a0f/nixexprs.tar.xz"
     },
     "release2111": {
-        "branch": "21.11",
+        "branch": "release-21.11",
         "description": "Nix Packages collection",
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "a7ecde854aee5c4c7cd6177f54a99d2c1ff28a31",
-        "sha256": "162dywda2dvfj1248afxc45kcrg83appjd0nmdb541hl7rnncf02",
+        "rev": "d887ac7aee92e8fc54dde9060d60d927afae9d69",
+        "sha256": "1bpgfv45b1yvrgpwdgc4fm4a6sav198yd41bsrvlmm3jn2wi6qx5",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/a7ecde854aee5c4c7cd6177f54a99d2c1ff28a31.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/d887ac7aee92e8fc54dde9060d60d927afae9d69.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "tahoe-lafs": {