Build: Explicitly fetch LFS files from fallback remote

Do it in make update tight after adding the remote. For some reason the
searchall mechanism does not work reliably immediately after checkout.

Fixes part of #139328.

build_files/utils/make_update.py

@@ -617,6 +617,9 @@ def lfs_fallback_setup(args: argparse.Namespace) -> None:
         push_url = "no_push"
         make_utils.git_add_remote(args.git_command, fallback_remote, url, push_url)
 
+        # Fetch potentially missing files.
+        call((args.git_command, "lfs", "fetch", fallback_remote))
+
 
 def main() -> int:
     args = parse_arguments()