Switch index-refinery from storing .tar.gz files to .tar.xz files

The `Homo_sapiens_short.tar.gz` file is 2.5 GB, whereas the same file compressed using `xz` is less than one GB. As these files don't actually feed into `salmon` in the gzipped format there's not much reason to use `gz` instead of `xz` other than the fact that `gz` may be easier for other users of the Index Refinery to use. However @jaclyn-taroni doesn't think that it would be a significant factor so `xz` seems superior.

No files, tests, or entry points are named. Locate the Index Refinery code that creates and stores the Homo_sapiens_short archive, then check how downstream consumers open it; the work is done when the archive is stored as .tar.xz without breaking supported consumers.

Written by the indexing model from the issue text.