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.

default.nix

@@ -3,7 +3,15 @@
 , compiler ? "ghc948"
 
 , # Commit of the Nixpkgs repository that we want to use.
-  nixpkgsVersion ? import nix/nixpkgs-version.nix
+  # It defaults to reading the inputs from flake.lock, which serves
+  # as a compatibility layer for non-flake builds / default.nix / shell.nix.
+  nixpkgsVersion ? let
+    lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  in
+  {
+    inherit (lock.nodes.nixpkgs.locked) owner repo rev;
+    tarballHash = lock.nodes.nixpkgs.locked.narHash;
+  }
 
 , # Nix files that describe the Nixpkgs repository. We evaluate the expression
   # using `import` below.
@@ -119,10 +127,6 @@ rec {
   memory =
     pkgs.callPackage nix/tools/memory.nix { inherit postgrestProfiled withTools; };
 
-  # Utility for updating the pinned version of Nixpkgs.
-  nixpkgsTools =
-    pkgs.callPackage nix/tools/nixpkgsTools.nix { };
-
   # Scripts for publishing new releases.
   release =
     pkgs.callPackage nix/tools/release.nix { };