Change I5fc96bf1f removed the generation of the .primary_build_tool
file and change I3d6b90320 removed the build system abstraction with
the motivation that we now only have one build tool.
However this broke launching the daemon from Eclipse on MacOS, which
now fails because bazel is not on the path and can't be found.
Restore the removed functionality, but rename the file to .bazel_path
Change-Id: Ibd0ec09d3bf47f383ca68a37cca4e81640960416
c26261acf9