Rebuilds genus_register.vtr and backend_coverage.vtr locally instead of
downloading the published pair. The build itself lives in taxifydb, which
resolves every backbone in the register's fixed backbone set and can take
well over an hour; the download takes seconds and yields the same file, so
this is an escape hatch rather than the normal route.
See also
taxify_load_register() to load it into memory,
taxify_register_coverage() to query backbone coverage for a genus.