We copy built tool binaries into a dedicated directory to avoid deleting
them, stageN-tools-bin. These aren't ever cleared out by code, since
there should be no reason to do so, and we'll simply overwrite them as
necessary.

When clearing out the stageN-{std,rustc,tools} directories, make sure to
delete both Cargo directories -- per-target and build scripts. This
ensures that changing libstd doesn't cause problems due to build scripts
not being rebuilt, even though they should be.

This makes it mandatory for other steps to have to handle the potential
failure instead of failing in an odd way later down the road.