Improve genrule sandboxing script

Do the sandboxed and non-sandboxed builds in two separate directories.
This allows us to keep the directories around so you can compare diffs
afterwards, and allows us to run the builds in parallel. It also means
that analysis isn't rerun twice every time you run the script.

Bug: 307824623
Test: Using it for the past few genrules I've fixed
Change-Id: Ib3be394f233b383c1bba41d31ada6c9af94e755b