Cleanup symbiflow-arch-defs build dir prior to terminating.
Cleanup symbiflow-arch-defs build dir prior to terminating.

By cleaning the symbiflow-arch-defs build dir, it prevents lots of small
files being transfered by kokoro, and cuts ~10 minutes off of the CI
runtime.

Signed-off-by: Keith Rothman <1bc19627a439baf17510dc2d0b2d250c96d445a5@users.noreply.github.com>

SymbiFlow/vtr-verilog-to-routing-kokoro