Declaratively trust nix-cache's SSH host key on remote-builder clients

Distributed builds failed with "Host key verification failed" on any
client that had never manually SSH'd to nix-cache before, since
nothing populated root's known_hosts for it. Wire nix-cache's host
public key into programs.ssh.knownHosts via a new vars.nixCacheHostKey
so every client picks it up automatically on rebuild.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

variables.nix

@@ -19,6 +19,15 @@
 
   remoteBuilderUser = "nixremote"; # remote builder SSH user
 
+  # nix-cache's own SSH host public key (not a secret — the private half
+  # never leaves the host). Wired into every client's
+  # programs.ssh.knownHosts by modules/nix-cache/remote-builder-client.nix
+  # so distributed builds don't hit "Host key verification failed" on a
+  # fresh client that has never manually ssh'd to nix-cache before. Update
+  # this if nix-cache's host key is ever rotated or the host is rebuilt
+  # from scratch.
+  nixCacheHostKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHrMKZlIGUd3pH9G3AqbsruqUGjxIXMAZw52u9MwiBCn lxc-nix-cache";
+
   # Public keys authorized to SSH in as remoteBuilderUser on the nix-cache
   # host (modules/nix-cache/server.nix) — one per client host that's allowed
   # to use it as a distributed builder.