[thegrebe]

Solves #101

Considerations

• wanted to avoid having duplicated BUSCO db (when one is already available on the system)
• wanted to avoid having funannotate2-related data in already-available BUSCO db
• wanted to avoid having to download the BUSCO db everytime at runtime when using Docker

Proposed solution

• if BUSCO_DOWNLOAD_PATH env variable is set, then use it
• if not, then fall back to current solution (FUNANNOTATE2_DB)