feat(ops/nixos): Add generic rebuild-system script

This adds a first crack at one idea for a generic, non-user-specific
rebuild-system script to ops.nixos.rebuild-system. The idea here is that
we enumerate all the nixos systems stored in the monorepo (similarly to
what we do for ci-builds right now) then search through them by hostname
to find the one matching the hostname of the current system, which is an
attempt at a more generic version of tazjin's rebuilder script which
does the same thing but with an explicit case block.

As a caveat, it feels like there's a slight possibility that this way of
finding systems is going to get slow to evaluate - on my system it feels
fine but if it grows out of hand it's probably feasible to just bake
this into the built script as a dynamically generated case statement.

Change-Id: I2e4c5401913b6f4d936ab48ba2f95f96e0e78eb4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/894
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>

ops/nixos/default.nix

@@ -6,8 +6,40 @@
 #
 # TODO(tazjin): Find a more elegant solution for the whole module
 # situation.
-{ ... }@args:
+{ lib, pkgs, depot, ... }@args:
 
-{
+let
+  inherit (lib) findFirst isAttrs;
+in
+
+rec {
   whitby = import ./whitby/default.nix args;
+
+  # System installation
+
+  allSystems = import ./all-systems.nix args;
+
+  nixosFor = configuration: depot.third_party.nixos {
+    inherit configuration;
+  };
+
+  findSystem = hostname:
+    (findFirst
+      (system: system.config.networking.hostName == hostname)
+      (throw "${hostname} is not a known NixOS host")
+      (map nixosFor allSystems));
+
+  rebuild-system = pkgs.writeShellScriptBin "rebuild-system" ''
+    set -ue
+    if [[ $EUID -ne 0 ]]; then
+      echo "Oh no! Only root is allowed to rebuild the system!" >&2
+      exit 1
+    fi
+
+    echo "Rebuilding NixOS for $HOSTNAME"
+    system=$(nix-build -E "((import ${toString depot.depotPath} {}).ops.nixos.findSystem \"$HOSTNAME\").system" --no-out-link --show-trace)
+
+    nix-env -p /nix/var/nix/profiles/system --set $system
+    $system/bin/switch-to-configuration switch
+  '';
 }