shell: add a nixpkgs-compat file

We are now able to pull in the same revision of nixpkgs as listed in
the lock file.

configuration.nix

@@ -5,6 +5,8 @@
 let
   inherit (builtins) attrNames readDir;
 
+  nixpkgs = toString (import ./nixpkgs-compat.nix);
+
   hostname = lib.fileContents /etc/hostname;
   host = "/etc/nixos/hosts/${hostname}.nix";
   config =
@@ -24,7 +26,7 @@ in
 
   networking.hostName = hostname;
   nix.nixPath = [
-    "nixpkgs=${<nixpkgs>}"
+    "nixpkgs=${nixpkgs}"
     "nixos-config=/etc/nixos/configuration.nix"
     "nixpkgs-overlays=/etc/nixos/overlays"
   ];

nixpkgs-compat.nix

@@ -0,0 +1,13 @@
+let
+  inherit (builtins)
+    fetchTarball
+    fromJSON
+    readFile
+    ;
+  nixos = (fromJSON (readFile ./flake.lock)).nodes.nixos.locked;
+  nixpkgs = fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${nixos.rev}.tar.gz";
+    sha256 = nixos.narHash;
+  };
+in
+nixpkgs

shell.nix

@@ -1,7 +1,4 @@
-{ pkgs ? import <nixpkgs> {
+{ pkgs ? import (import ./nixpkgs-compat.nix) { } }:
-    overlays = [ (import ./overlays/nix-zsh-completions.nix) ];
-  }
-}:
 let
   configs = "${toString ./.}#nixosConfigurations";
   build = "config.system.build";