Integrate usage of double-quoted strings in output path declarations

[simoxb]

At the moment, output paths in nf-core modules are specified in both single- and double-quotes. However, the usage of ' does not support the usual reference of Nextflow process variables (using ${var}). If using double-quoted strings becomes standard to improve consistency, it would make sense to integrate this here.

For more information, see nf-core/modules#7244.