Use nixpkgs 20.09

Author: hamishmack

nix/default.nix

@@ -4,7 +4,7 @@
 , sourcesOverride ? {}
 }:
 let
-  sources = import ./sources.nix { inherit pkgs; }
+  sources = import ./sources.nix { pkgs = import iohkNixMain.nixpkgs {}; }
     // sourcesOverride;
   iohkNixMain = import sources.iohk-nix {};
   haskellNix = (import sources."haskell.nix" { inherit system sourcesOverride; }).nixpkgsArgs;

nix/sources.json

@@ -34,5 +34,17 @@
         "type": "tarball",
         "url": "https://github.com/input-output-hk/niv/archive/89da7b2e7ae0779fd351618fc74df1b1da5e6214.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs": {
+        "branch": "nixpkgs-20.09-darwin",
+        "description": "Nix Packages collection",
+        "homepage": null,
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "07e5844fdf6fe99f41229d7392ce81cfe191bcfc",
+        "sha256": "0p2z6jidm4rlp2yjfl553q234swj1vxl8z0z8ra1hm61lfrlcmb9",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/07e5844fdf6fe99f41229d7392ce81cfe191bcfc.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }