In #114906, our docs CI builds began failing trying to build dashing from Go source.

We should pre-compile this to a binary vendor it via CIPD.

Note, this is generating a Dash docset, not the api docs: https://github.com/flutter/flutter/blob/master/dev/bots/docs.sh#L47.

We should also consider either not generating this docset (I'm not sure how many, if any, users it has), or separating it into a dependent step.