feat(midnight): add CI key for remote builds

We're remote building in CI to get around Tangled's limitations for nix
(and to avoid us having to figure out how to make nixery play nice with
a mounted nix daemon...)

Therefore, we'll need a remoteBuilds key that our Spindle can use

packetmix/systems/midnight/remoteBuilds.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+{
+  users.users.remoteBuilds.openssh.authorizedKeys.keys = [
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGVKhDwlrL4yXGSYrA8Pq40oNJukabt2mFe6ZRLub7oY patisserie-ci"
+  ];
+}