[Metagenomics Illumina] explicitly setting humann3 reference db locations in rule

[AstrobioMike]

• an issue can arise if the user is pointing to previously installed humann3 reference databases, or if the locations were manually changed for testing at some point
• this linked commit on the metagenomics branch integrated always explicitly setting the locations based on what is in the config.yaml: 1405949
• changes are in the humann3_PE and humann3_SE rules
• in case it's helpful, i'm also attaching a modified Snakefile that has the changes in it (as a zip due to file-type restrictions)
  Snakefile.zip