fix starfusion download output for gtf to refflat

[dodslaser]

Make output.chrgtf from STARFUSION_DOWNLOAD a tuple for compatibility with GTF_TO_REFFLAT.

This fixes reference generation when --starfusion_build is false.

PR checklist

• [x] This comment contains a description of changes (with reason).
• [x] Make sure your code lints (nf-core lint).
• [x] Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).