Wire configure-nix-cache-client.sh into create-proxmox-resource.sh's tooling bootstrap

Run it once, right after a node's first-time Nix bootstrap (not on every
invocation, and not inside codex-setup.sh/codex-maintenance.sh themselves),
so a freshly-bootstrapped Proxmox node substitutes from and can offload
builds to nix-cache on every subsequent run. Non-fatal on failure -- the
build still proceeds, just without nix-cache.

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

scripts/create-proxmox-resource.sh

@@ -524,7 +524,7 @@ ensure_remote_repo() {
   echo
   echo "==> Ensuring ${remote_repo_dir} exists and is current on ${node}..."
   if [[ "$dry_run" -eq 1 ]]; then
-    echo "[dry-run] would ensure ${remote_repo_dir} exists on ${node} (clone if missing, git pull if present), and would verify/bootstrap build tooling there (scripts/codex-setup.sh) if \`nix\` isn't already on PATH"
+    echo "[dry-run] would ensure ${remote_repo_dir} exists on ${node} (clone if missing, git pull if present), and would verify/bootstrap build tooling there (scripts/codex-setup.sh) if \`nix\` isn't already on PATH -- and if that bootstrap actually ran, would also configure ${node} as a nix-cache client (scripts/configure-nix-cache-client.sh)"
     return
   fi
 
@@ -571,6 +571,21 @@ ensure_remote_repo() {
   else
     echo "==> Bootstrapping build tooling on ${node} (scripts/codex-setup.sh)..."
     ssh "$ssh_target" "cd '${remote_repo_dir}' && bash scripts/codex-setup.sh"
+
+    # Only on this first-time bootstrap, not every run -- a node that
+    # already has tooling either already went through this once, or had
+    # it configured some other way, and re-running is harmless but
+    # pointless. Non-fatal: this only makes the node's own builds faster
+    # (substitute from nix-cache instead of building from source) and
+    # offloadable to it as a remote builder -- worth trying, not worth
+    # aborting the image build over if nix-cache happens to be down right
+    # now. Needs ensure_nix_profile first, same as the tooling_check_cmd
+    # above -- ssh's non-interactive command execution won't have picked
+    # up a freshly single-user-installed `nix` otherwise.
+    echo "==> Configuring ${node} as a nix-cache substituter/remote-builder client..."
+    if ! ssh "$ssh_target" "cd '${remote_repo_dir}' && . scripts/lib/nix-bootstrap.sh && ensure_nix_profile && bash scripts/configure-nix-cache-client.sh"; then
+      echo "WARNING: configure-nix-cache-client.sh failed on ${node} -- continuing without it (${node} will build from source / against cache.nixos.org only)." >&2
+    fi
   fi
 }