Stop re-using a stash with --reuse_sandbox_directories when the reuse is itself more expensive than creating a new sandbox directory

[oquenchil]

This affects for example test actions where two spawns are run, the first spawn runs the test and the second spawn processes the output to create an xml file.

The second will try to use the same stash as the first but will first have to clean it up, in some cases having to remove thousands of files that aren't needed.

[oquenchil]

For test actions (which have two spawns), special logic was already added to avoid reuse between the two different spawns. It's unlikely that this would provide any measurable benefit for regular actions.