Fix RBE tmp dir

The RBE tmp dir holds all the re-client logs and socket files.

Currently, it is being created as the `out/soong/.temp/` dir, which is auto deleted by Soong. This is causing problems because while a build is running, user cannot open a second terminal and run `lunch`.

This CL fixes the issue by creating the RBE tmp dir in the `out/soong/rbe/` directory. The next RBE build will clean this folder.


Bug: 349664018
Change-Id: Ib238e48f5b19755b016263b45f02b1be3e8efb35