testing sshless nix-cache copy

systems/jeeves/runners/nix_builder.nix

@@ -33,6 +33,10 @@ in
           mountPoint = "/zfs/media/github-runners/${name}";
           isReadOnly = false;
         };
+        "/host-nix" = {
+          mountPoint = "/nix";
+          isReadOnly = false;
+        };
         "/secrets".mountPoint = "${vars.secrets}/services/github-runners/${name}";
         "ssh-keys".mountPoint = "${vars.secrets}/services/github-runners/id_ed25519_github-runners";
       };