Prior to this change, multiplex sandboxed workers shared a working
directory per mnemonic. This caused a race condition when a new
multiplex sandboxed worker with the same mnemonic was launched. When
launching it cleaned the working directory, which could cause problems
for any actions executing in that directory.

This change makes it so each multiplex sandbox worker process has a
unique working directory. It does so while ensuring each
SandboxedWorkerProxy and its sandbox is still associated with the
correct multiplexer working directory.