When preparing ncov-tools-input directory, create symlinks instead of copies

[dfornika]

The prepare_data_root process creates copies of all alignments to pass into ncov-tools:

https://github.com/BCCDC-PHL/ncov-tools-nf/blob/66397d6b2714cb4810c5572ad615f855cfbaef20/modules/ncov-tools.nf#L74-L95

This has been acceptable for MiSeq runs, but is very slow for runs with high coverage, such as NextSeq runs.

Instead of creating copies in the prepare_data_root step, create symlinks.