Excise flakes in favour of npins and colmena

pinning.nix

@@ -0,0 +1,23 @@
+{ config, pkgs, ... }:
+let sources = import ./npins;
+in {
+  # https://jade.fyi/blog/pinning-nixos-with-npins/
+
+  # We need the flakes experimental feature to do the NIX_PATH thing cleanly
+  # below. Given that this is literally the default config for flake-based
+  # NixOS installations in the upcoming NixOS 24.05, future Nix/Lix releases
+  # will not get away with breaking it.
+  nix.settings.experimental-features = "nix-command flakes";
+
+  # FIXME(24.05 or nixos-unstable): change following two rules to
+  #
+  # nixpkgs.flake.source = sources.nixpkgs;
+  #
+  # which does the exact same thing, using the same machinery as flake configs
+  # do as of 24.05.
+  nix.registry.nixpkgs.to = {
+    type = "path";
+    path = sources.nixpkgs;
+  };
+  nix.nixPath = ["nixpkgs=flake:nixpkgs"];
+}