Add threads for for bowtie2-build

[chasemc]

The number of cpus is already passed to the alignment step (see ${task.cpus}) so pass it to the build step as well

$ bowtie2-build --help | grep "threads"
  --threads <int>         # of threads

The commands are here:

https://github.com/KwanLab/Autometa/blob/b1ec370c44e7accfc917aaad1a685133537fd915/modules/local/align_reads.nf#L37