Update downsampling subworkflow to samtools coverage

[LouisLeNezet]

PR checklist

The aim of this PR is to use samtools coverage instead of samtools depth.
The reason behind is that samtools depth tsv output can become quite huge (x10Gb) for whole genome data.
As we only need the mean depth samtools coverage can provided it if we give it the regions to downsampled.

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• Ensure that the test works with either Docker / Singularity. Conda CI tests can be quite flaky:
  • For modules:
    • nf-core modules test <MODULE> --profile docker
    • nf-core modules test <MODULE> --profile singularity
    • nf-core modules test <MODULE> --profile conda
  • For subworkflows:
    • nf-core subworkflows test <SUBWORKFLOW> --profile docker
    • nf-core subworkflows test <SUBWORKFLOW> --profile singularity
    • nf-core subworkflows test <SUBWORKFLOW> --profile conda