Fix sort paired reads

[hindrek]

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• This comment contains a description of changes (with reason).
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Problem:
Sorting forward and reverse FASTQ files independently can break paired samples.

Solution:
Generate reverse file paths by substituting '_1' -> '_2' in the forward files, ensuring perfect alignment of paired samples even when filenames are not lexicographically sortable.

Fixes #706

[nf-core-bot]

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