Fix ffq fallback for SRA downloads

[tsackton]

Summary

• replace the broken ffq --ftp xargs fallback in download_sra with a real shell loop
• log each ENA URL before downloading and send curl output into the Snakemake log
• add a dry-run regression assertion that the rendered SRA download command uses the looped fallback
• roll back sra-tools to 3.2.1 to fix segfaults

Testing

• pytest -q tests/tests.py -k mixed_srr_and_fastq_same_sample_dry_run