Add an option to pick a different build directory in minimise_corpuses.sh.
Also pass set -e instead of chaining things with &&. (One line was missing the
&&.)
Change-Id: Ia04e7f40f46688c9664101efefef1d1ea069de71
Reviewed-on: https://boringssl-review.googlesource.com/7300
Reviewed-by: Adam Langley <agl@google.com>