[build] Use GTest from brew in MacOS action.

[maxsharabayko]

Fixes the MacOS GitHub Action build error seen lately:

googletest-src/googlemock/include/gmock/gmock-actions.h:455:3: error: definition of implicit copy constructor for 'PolymorphicAction<testing::internal::ReturnNullAction>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
  GTEST_DISALLOW_ASSIGN_(PolymorphicAction);
  ^
/Users/runner/work/srt/srt/_build/googletest/googletest-src/googletest/include/gtest/internal/gtest-port.h:682:8: note: expanded from macro 'GTEST_DISALLOW_ASSIGN_'
  void operator=(type const &) = delete
       ^
/Users/runner/work/srt/srt/_build/googletest/googletest-src/googlemock/include/gmock/gmock-actions.h:[101](https://github.com/Haivision/srt/actions/runs/3877271627/jobs/6620352411#step:4:102)1:10: note: in implicit copy constructor for 'testing::PolymorphicAction<testing::internal::ReturnNullAction>' first required here
  return MakePolymorphicAction(internal::ReturnNullAction());
         ^
/Users/runner/work/srt/srt/_build/googletest/googletest-src/googlemock/include/gmock/gmock-actions.h:455:3: error: definition of implicit copy constructor for 'PolymorphicAction<testing::internal::ReturnVoidAction>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
  GTEST_DISALLOW_ASSIGN_(PolymorphicAction);
  ^
/Users/runner/work/srt/srt/_build/googletest/googletest-src/googletest/include/gtest/internal/gtest-port.h:682:8: note: expanded from macro 'GTEST_DISALLOW_ASSIGN_'
  void operator=(type const &) = delete
       ^
/Users/runner/work/srt/srt/_build/googletest/googletest-src/googlemock/include/gmock/gmock-actions.h:1016:10: note: in implicit copy constructor for 'testing::PolymorphicAction<testing::internal::ReturnVoidAction>' first required here
  return MakePolymorphicAction(internal::ReturnVoidAction());
         ^