Fix nix-cache retry storms and lxc creation in create-proxmox-resource.sh

Two independent problems found while actually running the script:

1. nix build/nix-shell retry each unreachable substituter/builder up to
   5x with backoff, per store path -- with nix-cache down this compounds
   into minutes of noise. scripts/env.sh gains nix_extra_opts(), which
   probes http://nix-cache and nixremote@nix-cache:22 once via plain
   curl/TCP (bypassing Nix's own retry logic entirely -- confirmed
   nix store ping still retries 5x even with a short connect-timeout)
   and exports the decision so create-proxmox-resource.sh and the
   sync-host-keys.sh subprocess it shells out to both reuse it instead
   of probing independently.

2. The actual failure: "archive contains no configuration file". pct
   restore expects a vzdump backup archive with embedded config;
   config.system.build.tarball is a plain CT template tarball -- wrong
   Proxmox mechanism entirely. Fixed to pct create against it as a vztmpl
   template instead, uploaded to /var/lib/vz/template/cache/ rather than
   /var/lib/vz/dump/. This same wrong claim had propagated into
   docs/auto-installer.md, README.md, and CLAUDE.md from when the script
   was first written -- corrected everywhere.

Also: checks for an already-uploaded image on the node (fixed
<flake_target>.tar.xz/.raw naming) before building, skipping build+upload
entirely if found (--force-rebuild to always rebuild).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

README.md

@@ -103,15 +103,15 @@ Three different paths depending on target, none of them involving a manual
   section covers how this stays non-destructive for `linode-*`, whose disks
   Linode itself provisions ahead of time).
 - `lxc-*` targets: not installed at all — build a ready-to-run container
-  tarball and `pct restore` it directly. `docs/auto-installer.md` covers why
-  (and the installer's menu excludes them for the same reason).
+  tarball and `pct create` it as a CT template directly. `docs/auto-installer.md`
+  covers why (and the installer's menu excludes them for the same reason).
 - `proxmox-*` targets: can alternatively be built as a standalone `.raw`
   disk image and attached to a new VM with no install step — see
   `docs/proxmox-images.md`.
 
 `scripts/create-proxmox-resource.sh --type lxc|vm --host <name>` automates
 either of the last two end to end (build, host-key registration, upload,
-`pct restore`/`qm create`), with `--dry-run` and a guard against duplicating
+`pct create`/`qm create`), with `--dry-run` and a guard against duplicating
 an already-deployed host's identity. See its `--help`.
 
 ## Security Notes