Add an extra check to needs_downloading for transcriptome index jobs.

[kurtwheeler]

Issue Number

Not sure, but this started breaking intermittently yesterday.

Purpose/Implementation Notes

Our transcriptome index jobs weren't handling a missing file quite correctly. This adds an additional check to needs_downloading to make sure they only recreate downloader jobs if they're the last processor job for the file.

Types of changes

• Bugfix (non-breaking change which fixes an issue)

Functional tests

The tests pass reliably!!!

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules