Fix lxc-* hosts having no host-key pre-seeding mechanism at all

The real root cause behind the original nix-cache 502, traced all the way
through: modules/installer/host-keys.nix (which NIXOS_HOST_KEYS_DIR=...
--impure actually wires up) is only ever imported by the installer's own
modules/installer/common.nix -- modules/platforms/lxc.nix, which every
real lxc-* host build actually uses, never imported anything like it.
docs/auto-installer.md previously claimed NIXOS_HOST_KEYS_DIR bakes a key
into lxc-* tarballs "the same way it does for the ISO/PXE installer
images" -- that was never actually true; I wrote it without verifying the
mechanism existed for lxc.nix specifically.

In practice this meant every lxc-* container booted with a freshly
self-generated SSH host key that could never match whatever .sops.yaml
actually trusts for that target, so *every* secret -- not just
cache-priv-key -- silently failed to decrypt. No error surfaces in the
boot log for this: the activation step that installs secrets only runs
on a genuinely fresh first activation and silently no-ops once
/run/current-system already exists, so by the time anyone looks the
window has closed. Found by manually invoking sops-install-secrets
directly: "Error getting data key: 0 successful groups required, got 0".

Fixed by giving modules/platforms/lxc.nix the same key-baking mechanism
the installer has, but keyed to its own exact flake target and placing
the key directly at /etc/ssh/ssh_host_ed25519_key (no copy step to stage
for, unlike the installer's /etc/host-keys/ staging area -- an lxc-*
tarball has no install step). The target name comes in via
specialArgs.flakeTarget (new, set by flake.nix's mkTarget) rather than
being read back from config.environment.etc."flake-target" -- reading
that back from within a module that also contributes to
environment.etc is circular (confirmed: "infinite recursion
encountered").

Verified live end-to-end against the real test container (lxc-nix-cache,
VMID 100 on pve.sweet.home): destroyed it, rebuilt the tarball fresh with
the fix, recreated it, and confirmed /run/secrets/ now has all three
secrets this host needs (beszel-token, cache-priv-key, nix-github-token),
nix-serve is active (running), and curl http://localhost/nix-cache-info
succeeds both directly and through nginx.

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

flake.nix

@@ -33,6 +33,9 @@
       # nix-cache itself consumes the nix-cache substituter and remote
       # builder.
       mkTarget = { platform, buildType, hostPath, homeFile ? ./modules/common/home.nix }:
+        let
+          flakeTarget = "${platform}-${buildType}";
+        in
         nixpkgs.lib.nixosSystem {
           inherit system;
           modules = [
@@ -42,7 +45,7 @@
             ./modules/platforms/${platform}.nix
             ./modules/build-types/${buildType}.nix
             hostPath
-            { environment.etc."flake-target".text = "${platform}-${buildType}"; }
+            { environment.etc."flake-target".text = flakeTarget; }
             home-manager.nixosModules.home-manager
             {
               home-manager = {
@@ -56,7 +59,13 @@
             ./modules/nix-cache/client.nix
             ./modules/nix-cache/remote-builder-client.nix
           ];
-          specialArgs = { inherit inputs vars netbootSystem; };
+          # flakeTarget is passed via specialArgs (not read back from
+          # config.environment.etc."flake-target" above) specifically so
+          # modules/platforms/lxc.nix can use it to select its own host key
+          # file without a same-option circular dependency (a module
+          # contributing to environment.etc can't read the merged
+          # environment.etc it's itself contributing to).
+          specialArgs = { inherit inputs vars netbootSystem flakeTarget; };
         };
 
       # Generated platform x build-type matrix. pxe-boot has no linode