Authorize this box's SSH key as a nix-cache remote-builder client

Following the existing per-client-key pattern in
vars.remoteBuilderAuthorizedKeys (one entry per host's own default root
key, not a shared dedicated keypair -- docs/nix-cache.md's "./nixremote"
filename is just a generic placeholder in the instructions). This box
had no entry yet, which is why its earlier `ssh-copy-id nixremote@nix-cache`
never stuck: nix-cache's nixremote authorized_keys is NixOS-declarative
and gets overwritten on every rebuild.

Public key only; requires a nix-cache rebuild to take effect.

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

variables.nix

@@ -38,6 +38,7 @@
     "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIGtOWOCS+ImHc7NehguoyD7PbonGosKMZqc9+QR3v/h root@nixos"
     "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHxXTQxFnArK5HXG7czeoybZebCGfxpUdusJkPn+BCSp root@server"
     "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICMJhrfFayLBG+gWtO6oAvgambw5nWWgztiTFEaaaVRH debian@surface"
+    "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJEj26SL/emsVjW2YhRucJVp2kTz8WgcEQgjBEBLRikk root@claude"
   ];
 
   # Admin SSH public key, authorized on the primary user of every host and