test(backgrounds): backdate the staging fixture through a write handle

`File::open` is read-only, and Windows backs `set_modified` with
`SetFileTime`, whose handle must carry `FILE_WRITE_ATTRIBUTES` —
`GENERIC_READ` does not include it. So the sweep test backdated its fixture
happily on Unix, where `futimens` gates an explicit timestamp on the file's
ownership rather than the descriptor's access mode, and failed the
windows-latest server cell with "Access is denied".