Use new bash script support for Songbird

Currently, we are calling Songbird from the rule itself. Would be better and consistent with the other methods if we made use of Snakemake's new(ish) support for bash scripts.

https://github.com/gibsramen/qadabra/blob/547f0732345d734c39fac63ba82b1b9b91a5b567/qadabra/workflow/rules/diffab.smk#L86-L99

https://snakemake.readthedocs.io/en/stable/snakefiles/rules.html#bash