nix: expose nixpkgs input on flake

This also moves the pin for nixpkgs into flake.lock instead of our
custom file. Even for the classic interface via default.nix, the pin
will be loaded from flake.lock, thus everything stays in-sync.

flake.nix

@@ -2,7 +2,7 @@
   description = "REST API for any Postgres database";
 
   inputs = {
-    nixpkgs-lib.url = "github:nix-community/nixpkgs.lib";
+    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
   };
 
   nixConfig = {
@@ -10,7 +10,7 @@
     extra-trusted-public-keys = "postgrest.cachix.org-1:icgW4R15fz1+LqvhPjt4EnX/r19AaqxiVV+1olwlZtI=";
   };
 
-  outputs = { nixpkgs-lib, ... }:
+  outputs = { nixpkgs, ... }:
     let
       systems = [
         "aarch64-darwin"
@@ -21,15 +21,21 @@
 
       pgrstFor = system: import ./default.nix {
         inherit system;
+        nixpkgsVersion = {
+          owner = "nixos";
+          repo = "nixpkgs";
+          inherit (nixpkgs) rev;
+          tarballHash = nixpkgs.narHash;
+        };
       };
 
-      genSystems = f: nixpkgs-lib.lib.genAttrs systems (system: f (pgrstFor system));
+      genSystems = f: nixpkgs.lib.genAttrs systems (system: f (pgrstFor system));
     in
     {
       packages = genSystems (attrs: {
         default = attrs.postgrestPackage;
         profiled = attrs.postgrestProfiled;
-      } // nixpkgs-lib.lib.optionalAttrs (attrs ? postgrestStatic) {
+      } // nixpkgs.lib.optionalAttrs (attrs ? postgrestStatic) {
         static = attrs.postgrestStatic;
       });