refactor: use nixpkgs from flake

Author: deemp

nix/nixpkgs.nix

@@ -1,8 +1,10 @@
+{ system ? builtins.currentSystem }:
 let
-  # nixpkgs is only used for development. Don't add it to the flake.lock.
-  gitRev = "2c2a09678ce2ce4125591ac4fe2f7dfaec7a609c";
+  lock = builtins.fromJSON (builtins.readFile ../flake.lock);
+  nixpkgs =
+    fetchTarball {
+      url = "https://github.com/NixOS/nixpkgs/archive/${lock.nodes.nixpkgs.locked.rev}.tar.gz";
+      sha256 = lock.nodes.nixpkgs.locked.narHash;
+    };
 in
-builtins.fetchTarball {
-  url = "https://github.com/NixOS/nixpkgs/archive/${gitRev}.tar.gz";
-  sha256 = "1pkz5bq8f5p9kxkq3142lrrq1592d7zdi75fqzrf02cl1xy2cwvn";
-}
+import nixpkgs { inherit system; }