rustbuild: Assert directory creation succeeds

rustbuild: Assert directory creation succeeds

I've been seeing failures on the bots when building jemalloc and my assumption
is that it's because cwd isn't created. That may be possible if this
`create_dir_all` call change in this commit fails, in which case we ignore the
error.

This commit updates the location to call `create_dir_racy` which handles
concurrent invocations, as multiple build scripts may be trying to create the
`native` dir.