BundleUtilities apparently doesn't do build-system-style timestamp
analysis when deciding what to copy, and instead just doesn't copy
things that are already present. This cleans that bundle directory on
each build, so that it includes the up-to-date library versions.

Since this is just copying from build artifacts, this is very fast; the
build steps themselves are not affected.

Fixes https://github.com/flutter/flutter/issues/58049