[gn build] Switch copy_bundle_data from pax to cpio

This will hopefully fix the build not becoming clean when using Ninja
1.9+. Ninja 1.9 enabled high-resolution time stamps, but pax doesn't
correctly set high-resolution timestamps on its output.

See https://github.com/nico/hack/blob/master/notes/copydir.md for a
detailed writeup of problem and alternatives.