(nix): use unstable nixpkgs for dev-environment

flake.lock

@@ -31,10 +31,27 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1606725183,
+        "narHash": "sha256-QIdgDJUnT/Gq8vURmQqy1tTVpI0cWryoD9/auCAkQbE=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "24eb3f87fc610f18de7076aee7c5a84ac5591e3e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
         "import-cargo": "import-cargo",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nixpkgs-unstable": "nixpkgs-unstable"
       }
     }
   },

flake.nix

@@ -2,11 +2,12 @@
     description = "Converts audio files";
 
     inputs = {
-        nixpkgs.url = github:NixOS/nixpkgs/nixos-20.09;
         import-cargo.url = github:edolstra/import-cargo;
+        nixpkgs.url = github:NixOS/nixpkgs/nixos-20.09;
+        nixpkgs-unstable.url = github:NixOS/nixpkgs/nixos-unstable;
     };
 
-    outputs = { self, nixpkgs, import-cargo }:
+    outputs = { self, nixpkgs, nixpkgs-unstable, import-cargo }:
         let
             inherit (import-cargo.builders) importCargo;
         in {
@@ -47,7 +48,7 @@
                 };
 
             devShell.x86_64-linux =
-                with import nixpkgs { system = "x86_64-linux"; };
+                with import nixpkgs-unstable { system = "x86_64-linux"; };
                 stdenv.mkDerivation {
                     name = "audio-conv";
                     buildInputs = [