Adds debian@surface's existing SSH key (already used as its admin
key elsewhere in this repo) to vars.remoteBuilderAuthorizedKeys so
nix-cache will accept it as a distributed-build client once deployed.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot
