--experimental_sandbox_async_tree_delete_idle_threads=<integer, or a keyword ("auto", "HOST_CPUS", "HOST_RAM"), optionally followed by an operation ([-|*]<float>) eg. "auto", "HOST_CPUS*.5">
If 0, delete sandbox trees as soon as an action completes (causing
completion of the action to be delayed). If greater than zero, execute the
deletion of such threes on an asynchronous thread pool that has size 1 when
the build is running and grows to the size specified by this flag when the
server is idle.