Prevent temporary file conflict

[alexheretic]

Though it's less usual to run concurrently against the same file I think this should just work anyway. You may, for example, want to try multiple different encoder args for an encoder than uses a ~1 thread on a multi-thread machine.

Currently the temporary dir is shared. Since you can specify --temp-dir conflicts can already by worked around. But it would probably be better if conflicts didn't happen out of the box.

A simple solution is to use a random named nested temp-dir inside --temp-dir for each run.