Use new bash script support for Songbird

[gibsramen]

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