Sharded map reduce may fail to clean up temporary output collection

This call is supposed to drop the temporary output collections if anything goes wrong in the command, but in some cases we don't reach there. In particular, I've reproduced a failure where this line can throw an exception due to a stale config. When that happens, the cleanup code doesn't run and the collection is left around.

To fix this, we should use a ScopeGuard to make sure we clean up the temporary collections if we exit the function due to an exception.