The Mac grpc-tools build is broken with a CMake error

[murgatroid99]

See this job run for an example failure.

Error message:

CMake Error at deps/protobuf/cmake/CMakeLists.txt:2 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.
-- Configuring incomplete, errors occurred!

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.