Make search path for FindPkgConfig explicit in FindTBB

Summary

We have included a backport of FindPkgConfig.cmake in the cmake/modules/FindPkgConfig folder that was added to CMAKE_MODULE_PATH whenever DuneMacros.cmake is included. This behavior might be an issue if a project does not include DuneMacros.cmake or appends another directory including another FindPkgConfig.cmake file to CMAKE_MODULE_PATH. This MR proposes to modify the global search path explicitly before the find_package(PkgConfig) is called.

NOTE: This MR puts the directory of the backports at the front of the CMAKE_MODULE_PATH list, in order to prefer this directory over others added at the end.