Update flake definition.

flake.lock

@@ -32,11 +32,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1610842533,
-        "narHash": "sha256-6hW8CML8RnNrRJMv7E56rXAhsCNgUM97HIVSqWxnO64=",
+        "lastModified": 1612194028,
+        "narHash": "sha256-AFPVC9gNDHnx52FGD1IWRvM3S3D1AgEbEaOkxDerlyU=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "68398d2dd50efc2d878bf0f83bbc8bc323b6b0e0",
+        "rev": "65211f5afcc3637c55423b327157a5eae05dff67",
         "type": "github"
       },
       "original": {
@@ -82,6 +82,7 @@
       "original": {
         "owner": "oxalica",
         "repo": "rust-overlay",
+        "rev": "a9b13ba83eaf2d07ae955a45b15fd96aa6994b70",
         "type": "github"
       }
     }

flake.nix

@@ -4,10 +4,10 @@
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
-    rust-overlay.url = "github:oxalica/rust-overlay";
+    rust-overlay.url = "github:oxalica/rust-overlay/a9b13ba83eaf2d07ae955a45b15fd96aa6994b70";
   };
 
-  outputs = { nixpkgs, rust-overlay, flake-utils, ... }:
+  outputs = inputs@{ self, nixpkgs, rust-overlay, flake-utils, ... }:
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs { inherit system; overlays = [ rust-overlay.overlay ]; };

shell.nix

@@ -1,4 +1,4 @@
-{ stdenv, pkgs ? import <nixpkgs> {} }:
+{ stdenv, pkgs }:
 
 pkgs.mkShell {
   nativeBuildInputs = with pkgs; [