Fix falco sample names

[MartinGrignard]

Hi 👋

This PR modifies the definition of the falco module so that the reported sample names are affected by the value of the prefix variable instead of the actual input file name.
To achieve this, it simply uses the same strategy as many other modules: Symlinks are now created to provide input files named after the desired prefix.
This makes it easier to include falco output in MultiQC reports.

In the meantime, I also updated the value of falco form 1.2.1 to 1.2.5.

[SPPearce]

You'll need to regenerate the test snapshots (nf-core modules test falco --update) and update the environment.yml to change the conda module version.