Authorize the WSL workstation as a nix-cache remote-builder client

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

flake.nix

@@ -56,7 +56,7 @@
             ./modules/nix-cache/client.nix
             ./modules/nix-cache/remote-builder-client.nix
           ];
-          specialArgs = { inherit inputs vars; };
+          specialArgs = { inherit inputs vars netbootSystem; };
         };
 
       # Generated platform x build-type matrix. pxe-boot has no linode