lookup_beam.py being run twice

[nhurleywalker]

I don't think this second call of lookup_beam.py is necessary, since it's already run with the same settings at the start of the script. At the very least, we could add a check to see whether the beams already exist in order to avoid wasting time regenerating them.

https://github.com/GLEAM-X/GLEAM-X-pipeline/blob/6f14641e553c03adec321e47ca7aa35a062bc216/templates/postimage.tmpl#L265C1-L280C31