fix(server): disable zsh new user welcome

We can do this by redefining the function zsh calls to be a no-op...
I haven't decided if we want this on user machines, but we definitely
don't want it on servers

Refs: https://discourse.nixos.org/t/zsh-configuration-for-new-users-keeps-recurring

packetmix/systems/server/zsh.nix

···
+# SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+#
+# SPDX-License-Identifier: MIT
+
+{
+  programs.zsh.shellInit = "zsh-newuser-install() { true }";
+}

packetmix/workspace.josh.license

···
+SPDX-FileCopyrightText: 2025 FreshlyBakedCake
+
+SPDX-License-Identifier: CC0-1.0