ExternalProject: Always use CMake builtin FindGit (#15791)

Unset CMAKE_MODULE_PATH before calling find_package(Git) to ensure we do
not get a project-provided FindGit module that may not provide the
GIT_VERSION_STRING that we need.  We do not need to restore the value
explicitly because this occurs inside the _ep_add_download_command
function call scope.

Modules/ExternalProject.cmake

@@ -1731,6 +1731,7 @@ function(_ep_add_download_command name)
       --non-interactive ${svn_trust_cert_args} ${svn_user_pw_args} ${src_name})
     list(APPEND depends ${stamp_dir}/${name}-svninfo.txt)
   elseif(git_repository)
+    unset(CMAKE_MODULE_PATH) # Use CMake builtin find module
     find_package(Git QUIET)
     if(NOT GIT_EXECUTABLE)
       message(FATAL_ERROR "error: could not find git for clone of ${name}")