flake: add nixpkgs-unstable overlay

flake.lock

@@ -16,9 +16,26 @@
         "type": "github"
       }
     },
+    "nixpkgs-unstable": {
+      "locked": {
+        "lastModified": 1613226215,
+        "narHash": "sha256-3rA5cGIrBHD6yeKhNhsF7/t461ww25oJY8KyBb0IhjU=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "ff96a0fa5635770390b184ae74debea75c3fd534",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "nixpkgs-unstable": "nixpkgs-unstable"
       }
     }
   },

flake.nix

@@ -2,16 +2,23 @@
   description = "Nixos configuration with flakes";
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-20.09-small";
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
   };
 
-  outputs = { self, nixpkgs }: {
-    nixosConfigurations.poseidon = nixpkgs.lib.nixosSystem {
+  outputs = { self, nixpkgs, nixpkgs-unstable }: {
+    nixosConfigurations.poseidon = nixpkgs.lib.nixosSystem rec {
       system = "x86_64-linux";
       modules =
         [
           ./poseidon.nix
+
           {
-            nixpkgs.overlays = import ./overlays;
+            nixpkgs.overlays = [
+              # packages accessible through pkgs.unstable.package
+              (final: prev: {
+                unstable = nixpkgs-unstable.legacyPackages.${system};
+              })
+            ] ++ (import ./overlays);
           }
         ];
     };