Use threads option for zstd

libarchive does support the --threads flag for zstd, although this is missing in the man pages.

See https://github.com/libarchive/libarchive/blob/04be21ea4d973c2c80a1d456e0ea3fa3851e9174/libarchive/archive_write_add_filter_zstd.c#L433 and https://github.com/libarchive/libarchive/blob/04be21ea4d973c2c80a1d456e0ea3fa3851e9174/libarchive/archive_write_add_filter_zstd.c#L219 for the implementation.