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>

docs/nix-cache.md

@@ -59,6 +59,15 @@ On `nix-cache`, install the matching public key used by `nixremote` authorized k
 The committed `nixremote` authorized keys are public SSH keys only. Keep the
 matching private keys on client hosts and out of the repository.
 
+nix-cache's own SSH *host* key is trusted declaratively via
+`programs.ssh.knownHosts` in `modules/nix-cache/remote-builder-client.nix`,
+sourced from `vars.nixCacheHostKey` (`variables.nix`) — every client rebuild
+picks it up automatically, so distributed builds don't fail with "Host key
+verification failed" on a client that has never manually SSH'd to nix-cache
+before. If nix-cache's host key is ever rotated or the host rebuilt from
+scratch, update `vars.nixCacheHostKey` to match its new
+`/etc/ssh/ssh_host_ed25519_key.pub`.
+
 ## Manual verification
 
 After deployment: