Allow specifying binary BEP in another location than /tmp/bep.bep

[jerrymarino]

The BazelBuildService example currently requires hardcoding by adding the param to their project's .bazelrc or ~/.bazelrc and it doesn't support multiple bazel builds at a time.

It needs more effort to set the param and or run Bazel directly. This is a good time to consider core features including:

• Calling Bazel directly from here and dropping XCBuild's logic. Users currently have a runscript phase that invoke Bazel. Currently Tulsi and XCHammer both pass a temp JSON BEP path to Bazel.

• BEP vs build event service.