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

docs/auto-installer.md

@@ -34,10 +34,26 @@ container image, no install step at all:
 nix build .#nixosConfigurations.lxc-minimal.config.system.build.tarball
 ```
 
-Then, on the Proxmox host, `pct restore` (or the GUI's "Create CT" → upload
-template flow) that tarball directly as a new container. First boot runs
-`boot.postBootCommands` (registers the Nix store DB and system profile) —
-there's no separate activation step to run yourself.
+This is a plain rootfs tarball, not a `vzdump` backup archive — restoring it
+with `pct restore` fails ("archive contains no configuration file"), since
+that command expects backup-archive metadata this tarball doesn't have. Use
+it as a CT *template* instead: drop it under Proxmox's template storage
+(conventionally `/var/lib/vz/template/cache/` for the `local` storage, or
+the GUI's "Create CT" → upload-as-template flow) and create a container
+from it, supplying all config on the command line since a template has none
+of its own:
+
+```sh
+pct create <vmid> local:vztmpl/<file>.tar.xz \
+  --rootfs local-lvm:8 --hostname <name> --cores 2 --memory 2048 \
+  --net0 name=eth0,bridge=vmbr0,ip=dhcp
+pct start <vmid>
+```
+
+First boot runs `boot.postBootCommands` (registers the Nix store DB and
+system profile) — there's no separate activation step to run yourself.
+`scripts/create-proxmox-resource.sh --type lxc --host <name>` automates all
+of this (build, host-key handling, upload, `pct create`) — see its `--help`.
 
 Host keys still need pre-seeding the same way as any other host (see "Host
 keys" below) — the sops-nix activation-vs-first-boot race is identical