[build] Put generated.h back in snapshot dir

This is where users of the flatbuffer build rule expects snapshots are
stored. For example:
https://cs.opensource.google/fuchsia/fuchsia/+/master:src/ui/scenic/lib/gfx/snapshot/serializer.h;l=9;drc=ed53a56dfcb5e30e00894691ae2748636cd5685e

This is a followup to 194045e, didn't notice this problem until a
manual roll attempt failed CQ. Not sure why it didn't manifest when I
was testing locally, most likely due to I'm not doing a proper clean.
Did a more thorough local test this time.

Test:
rm -rf out
fx set core.x64 \
  --with //bundles:build_infra,//bundles:test_infra \
  --args=build_should_trace_actions=true
fx build

Change-Id: I972a5ba6e48f1bed883b3896ba0dc6218529bcdb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/472661
Reviewed-by: Shai Barack <shayba@google.com>