nixos/systemd-boot-builder: write loader.conf after the entries it points at

A crash between the two would leave `default <id>` referring to a
.conf that does not exist yet.

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

@@ -540,8 +540,6 @@ def install_bootloader(args: argparse.Namespace) -> None:
             if is_default:
                 default_entry_id = new_bootctl_id
 
-    write_loader_conf(default_entry_id)
-
     # Garbage-collect stale kernels/initrds/entries before re-populating extra
     # files, so that user-supplied extraEntries (which may also live under
     # loader/entries and start with `nixos-`) are not removed again.
@@ -549,6 +547,8 @@ def install_bootloader(args: argparse.Namespace) -> None:
 
     write_boot_files(boot_files)
 
+    write_loader_conf(default_entry_id)
+
     remove_extra_files()
     run([COPY_EXTRA_FILES])