use cache-dir for arti storage

I used to get a weird, very non-specific error: A/libc: Fatal signal 6 (SIGABRT), code -1 (SI_QUEUE) in tid 20564 (Thread-3), pid 20367 (ject.artitoyvpn)

Turns out it happens because arti_builder.create_unbootstrapped was returning and Err(_) because it was not given a directory to write to, and that error was being .unwraped, but having a panic unwind into the Java runtime leads to immediate SIGABRT. I know this is just a prototype yet, but care should be taken either to not panic, or catch unwind in extern "C" functions.