perf(dask): use global functions instead of lambdas where possible

[kszucs]

          When possible, it's better to use top-level functions instead of lambdas for cheaper pickling of tasks. This is probably negligible here, but it's good to keep in mind.

Originally posted by @jcrist in https://github.com/ibis-project/ibis/pull/8005#discussion_r1473084588