nixos/container-config: Only use true as fallback

LXC containers like those used by VPSAdminOS might want to install a bootloader
so passing `true` to `system.build.installBootLoader` without any priority specified,
causes a conflict for such systems with the recent `system.build` changes:

https://github.com/NixOS/nixpkgs/commit/4014fb6a64bc5f68326fc08cbaa83475db1fae8e

Fixes: https://github.com/NixOS/nixpkgs/issues/155839

nixos/modules/virtualisation/container-config.nix

@@ -18,7 +18,7 @@ with lib;
     services.openssh.startWhenNeeded = mkDefault true;
 
     # Shut up warnings about not having a boot loader.
-    system.build.installBootLoader = "${pkgs.coreutils}/bin/true";
+    system.build.installBootLoader = lib.mkDefault "${pkgs.coreutils}/bin/true";
 
     # Not supported in systemd-nspawn containers.
     security.audit.enable = false;