Fix auto-install.sh to work standalone, not just baked into the image

Two real bugs, both hit live:

1. Shebang: #!/run/current-system/sw/bin/bash only resolves on an
   already-activated NixOS system -- running the checked-out script
   directly (e.g. from a stock ISO, cloned repo) failed with "cannot
   execute: required file not found" on a non-NixOS box. Switched to
   #!/usr/bin/env bash, which resolves identically on NixOS
   (environment.usrbinenv's own default) and any normal Linux distro.
   Also fixed the file's missing executable bit.

2. FLAKE_BASE_URL: previously depended on pkgs.replaceVars substituting
   a Nix-templated @lanDomain@ placeholder at build time -- meaning it
   only ever worked when baked into the built installer image, not when
   run straight from a checkout (the literal, unexpanded "@lanDomain@"
   string reached git as a bogus hostname). Replaced with LAN_DOMAIN in
   scripts/env.sh (manually kept in sync with variables.nix's lanDomain,
   same pattern as NIX_CACHE_HOST/nixCacheHost already), sourced by the
   script itself like every other script in scripts/. Dropped
   pkgs.replaceVars from modules/installer/common.nix entirely --
   scripts/env.sh is now baked into the image alongside auto-install.sh
   at a matching relative path (/etc/nixos-installer/env.sh next to
   /etc/nixos-installer/installer/auto-install.sh) so the script's own
   relative `source` line resolves the same way in both contexts.

loginShellInit's invocation path and docs/auto-installer.md updated to
match. Verified: shellcheck clean on both scripts, the baked files are
byte-identical to their checked-in sources (no templating left to
verify), and codex-maintenance.sh (secret grep, fmt, statix, full eval
of every host/package including the installer/pxe artifacts) passes
clean.

scripts/installer/auto-install.sh

@@ -1,9 +1,12 @@
-#!/run/current-system/sw/bin/bash
+#!/usr/bin/env bash
 set -eux
 
 set -euo pipefail
 
-export FLAKE_BASE_URL="git+https://@lanDomain@/beatzaplenty/nixos.git"
+# shellcheck source=../env.sh
+source "$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)/env.sh"
+
+export FLAKE_BASE_URL="git+https://${LAN_DOMAIN}/beatzaplenty/nixos.git"
 
 echo "Fetching available NixOS hosts from flake..."
 # Two categories deliberately excluded from the menu: