The Sawzall tests expect tools in /usr/local.

This commit instructs Sawzall to find the Protocol Buffer tools under
the local build and testing sandbox.  To is to ensure hermeticness
of the test environment.  Without this, the Sawzall tool will not find
our tools due to their not being installed under the hardcoded path of
/usr/local/bin.

Let's wait to merge until [![Build Status](https://travis-ci.org/arbeitspferde/sawzall.png?branch=fix/testing/protocol-buffer-tools-path)](https://travis-ci.org/arbeitspferde/sawzall).