Centralize shared values into variables.nix

One file (variables.nix) holding every value that was previously
hardcoded and repeated across modules: LAN domain/CIDR, home/tailnet
domains, cross-host references (nix-cache substituter hostname, NFS
server hostname, remote-builder user), PXE/PBS IPs, timezone, and the
primary username.

Wired in via flake.nix's specialArgs (and home-manager's
extraSpecialArgs for the two home.nix files), so any module picks it
up by just adding `vars` to its function arguments — no explicit
import needed. Two hosts (nix-cache, server) now derive their own
networking.hostName from the same variable other hosts use to reach
them, so there's exactly one place to change either identifier.

Purely mechanical: every substituted value matches what was already
there, confirmed by identical toplevel .drv paths for all 17 targets
before and after.

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

modules/remote-builder-client.nix

@@ -1,4 +1,4 @@
-{ pkgs, ... }:
+{ pkgs, vars, ... }:
 
 {
   # Install the remote builder key on each client host (do not commit private keys):
@@ -9,9 +9,9 @@
 
   nix.buildMachines = [
     {
-      hostName = "nix-cache";
-      sshUser = "nixremote";
-      sshKey = "/root/.ssh/nixremote";
+      hostName = vars.nixCacheHost;
+      sshUser = vars.remoteBuilderUser;
+      sshKey = "/root/.ssh/${vars.remoteBuilderUser}";
       system = pkgs.stdenv.hostPlatform.system;
       maxJobs = 4;
       speedFactor = 2;