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>

CLAUDE.md

@@ -97,7 +97,12 @@ Beyond `codex-setup.sh`/`codex-maintenance.sh` above, `scripts/` also has:
   Refuses to create a target whose host identity already exists live on
   the node (checked directly via `qm`/`pct`, not any file in this repo)
   unless `--allow-duplicate-host` is passed. `--dry-run` throughout both
-  modes.
+  modes. The first time it has to bootstrap build tooling on a node (i.e.
+  `nix` wasn't already on its `PATH`), it also runs
+  `scripts/configure-nix-cache-client.sh` there (non-fatally — a failure
+  just falls back to building from source / `cache.nixos.org`) so the
+  node substitutes from and can offload builds to nix-cache on every
+  subsequent run, not just this one.
 - `scripts/env.sh` — shared config (`PROXMOX_HOST`, storage pool, bridge,
   default cores/memory) sourced by `create-proxmox-resource.sh`. Add new
   cross-script config here instead of duplicating it per-script.