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.

nix/tools/nixpkgsTools.nix

@@ -1,62 +0,0 @@
-{ buildToolbox
-, checkedShellScript
-, coreutils
-, curl
-, jq
-, nix
-}:
-# Utility script for pinning the latest stable version of Nixpkgs.
-
-# Instead of pinning Nixpkgs based on the huge Git repository, we reference a
-# specific tarball that only contains the source of the revision that we want
-# to pin.
-let
-  name =
-    "postgrest-nixpkgs-upgrade";
-
-  refUrl =
-    "https://api.github.com/repos/nixos/nixpkgs/git/matching-refs/heads/nixpkgs-";
-
-  githubV3Header =
-    "Accept: application/vnd.github.v3+json";
-
-  tarballUrlBase =
-    "https://github.com/nixos/nixpkgs/archive/";
-
-  upgrade =
-    checkedShellScript
-      {
-        inherit name;
-        docs = "Pin the newest stable version of Nixpkgs.";
-        workingDir = "/";
-      }
-      ''
-        # The list of refs is sorted. The first result will be nixpkgs-unstable, the second the latest stable branch.
-        # shellcheck disable=SC2207
-        commit=($(${curl}/bin/curl "${refUrl}" -H "${githubV3Header}" | ${jq}/bin/jq -r 'sort_by(.ref) | reverse | [.[0].object.sha, .[0].ref] | @tsv'))
-        tarballUrl="${tarballUrlBase}''${commit[0]}.tar.gz"
-        tarballHash="$(${nix}/bin/nix-prefetch-url --unpack "$tarballUrl")"
-        currentDate="$(${coreutils}/bin/date --iso)"
-
-        cat > nix/nixpkgs-version.nix << EOF
-        # Pinned version of Nixpkgs, generated with ${name}.
-        {
-          owner = "NixOS";
-          repo = "nixpkgs";
-          ref = "''${commit[1]}";
-          date = "$currentDate";
-          rev = "''${commit[0]}";
-          tarballHash = "$tarballHash";
-        }
-        EOF
-
-        echo "# This file is auto-generated by ${name}" > docs/requirements.txt
-        cat "$(nix-build -A docs.requirements)" >> docs/requirements.txt
-      '';
-
-in
-buildToolbox
-{
-  name = "postgrest-nixpkgs";
-  tools = { inherit upgrade; };
-}