Fix problem if process.stageOutMode = 'move'

[Lehmann-Fabian]

Using process.stageOutMode = 'move' will only move files that match the output from a temporary directory to the working directory. Here, it breaks the symlink ${prefix}.bam -> $bam as only ${prefix}.bam is moved, creating a dead link.
Defining path(bam) as output solves this problem.

PR checklist

• This comment contains a description of changes (with reason).
• Ensure the test suite passes (nextflow run . -profile test,docker --outdir <OUTDIR>).
• CHANGELOG.md is updated.