Don't send ldd/otool output to the same file.

This will allow multiple omnibus project builds to run concurrently
(with separate cache dirs).

* GC concern in the original comment is addressed in mixlib-shellout 1.4
* it should be fine holding entire shell output in memory as it's doing it already.
  currently, it redirect all output to disk and then read the entire file
  with a File.read call. so with the change, we actually save the trip to disk.