refactor(ops/nixos): Move my NixOS configurations to //users/tazjin

NixOS modules move one level up because it's unlikely that //ops/nixos
will contain actual systems at this point (they're user-specific).

This is the first users folder, so it is also added to the root
readTree invocation for the repository.

Change-Id: I546c701145fa204b7ba7518a8a56a783588629e0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/244
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/nixos/default.nix

@@ -0,0 +1,46 @@
+# TODO(tazjin): Generalise this and move to //ops/nixos
+{ depot, lib, ... }:
+
+let
+  inherit (builtins) foldl';
+
+  systemFor = configs: (depot.third_party.nixos {
+    configuration = lib.fix(config:
+      foldl' lib.recursiveUpdate {} (map (c: c config) configs)
+    );
+  }).system;
+
+  caseFor = hostname: ''
+    ${hostname})
+      echo "Rebuilding NixOS for //users/tazjin/nixos/${hostname}"
+      system=$(nix-build -E '(import <depot> {}).users.tazjin.nixos.${hostname}System' --no-out-link)
+      ;;
+  '';
+
+  rebuilder = depot.third_party.writeShellScriptBin "rebuilder" ''
+    set -ue
+    if [[ $EUID -ne 0 ]]; then
+      echo "Oh no! Only root is allowed to rebuild the system!" >&2
+      exit 1
+    fi
+
+    case $HOSTNAME in
+    ${caseFor "nugget"}
+    ${caseFor "camden"}
+    ${caseFor "frog"}
+    *)
+      echo "$HOSTNAME is not a known NixOS host!" >&2
+      exit 1
+      ;;
+    esac
+
+    nix-env -p /nix/var/nix/profiles/system --set $system
+    $system/bin/switch-to-configuration switch
+  '';
+in {
+  inherit rebuilder;
+
+  nuggetSystem = systemFor [ depot.users.tazjin.nixos.nugget ];
+  camdenSystem = systemFor [ depot.users.tazjin.nixos.camden ];
+  frogSystem = systemFor [ depot.users.tazjin.nixos.frog ];
+}