feat: use nix-output-monitor for builds

It'd be nice to use nix output monitor for builds - there's a project
which automatically uses nix output monitor when nix is called (even
supporting nix-direnv/nixos-rebuild)... this looks probably good for us
to use.

There is [one issue](https://github.com/ners/nix-monitored/issues/3)
of some importance on the nix-monitored tracker which suggests that
nix-monitored incorrectly transforms some 'nix run' invocations...
fortunately, we don't use flakes so this shouldn't be a huge issue

If it is, using

NIX_MONITOR=disable

will temporarily disable nix monitor, allowing you to run whatever you
needed without issue while using nom for everything else

packetmix/npins/sources.json

···
       "url": "https://github.com/nix-community/nix-index-database/archive/050a5feb5d1bb5b6e5fc04a7d3d816923a87c9ea.tar.gz",
       "hash": "sha256-3HIpe3P2h1AUPYcAH9cjuX0tZOqJpX01c0iDwoUYNZ8="
     },
+    "nix-monitored": {
+      "type": "Git",
+      "repository": {
+        "type": "GitHub",
+        "owner": "ners",
+        "repo": "nix-monitored"
+      },
+      "branch": "master",
+      "submodules": false,
+      "revision": "60f3baa4701d58eab86c2d1d9c3d7e820074d461",
+      "url": "https://github.com/ners/nix-monitored/archive/60f3baa4701d58eab86c2d1d9c3d7e820074d461.tar.gz",
+      "hash": "sha256-Z8PknjkmIr/8ZCH+dmc2Pc+UltiOr7/oKg37PXuVvuU="
+    },
     "nixos-unstable": {
       "type": "Channel",
       "name": "nixos-unstable",

packetmix/systems/common/lix.nix

···
 #
 # SPDX-License-Identifier: MIT
 
-{ project, ... }:
+{
+  project,
+  pkgs,
+  system,
+  ...
+}:
 {
   imports = [
     (import "${project.inputs.lix-module.result}/module.nix" { lix = project.inputs.lix.src; })
···
     options = "--delete-older-than 7d";
     dates = "08:30";
   };
+
+  nixpkgs.overlays = [
+    (final: prev: {
+      nix-monitored = project.inputs.nix-monitored.result.packages.${system}.default.override {
+        nix = final.lix;
+        withNotify = pkgs.stdenv.isLinux;
+      };
+      nixos-rebuild = prev.nixos-rebuild.override {
+        nix = final.nix-monitored;
+      };
+      nix-direnv = prev.nix-direnv.override {
+        nix = final.nix-monitored;
+      };
+    })
+  ];
+
+  nix.package = pkgs.nix-monitored;
 }