[ROCm][MIGraphX] for googletest dep, set OVERRIDE_FIND_PACKAGE (#16715)

Otherwise, an unsupported version of gtest/gmock will be found at
/opt/conda/include for ROCm builds. Though this issue was initially
found for ROCm builds, the issue is generic. onnxruntime requires a
specific version of googletest and should not rely on locating
googletest using find_package.

The ROCm error was:

```
In file included from /opt/conda/include/gmock/gmock-spec-builders.h:75,
                 from /opt/conda/include/gmock/gmock-generated-function-mockers.h:47,
                 from /opt/conda/include/gmock/gmock-function-mocker.h:39,
                 from /opt/conda/include/gmock/gmock.h:61,
                 from /stage/onnxruntime/onnxruntime/test/util/test_utils.cc:17:
/opt/conda/include/gmock/gmock-matchers.h: In instantiation of ‘bool testing::internal::PointwiseMatcher<TupleMatcher, RhsContainer>::Impl<LhsContainer>::
MatchAndExplain(LhsContainer, testing::MatchResultListener*) const [with LhsContainer = const gsl::span<const float>&; TupleMatcher = testing::internal::
FloatingEq2Matcher<float>; RhsContainer = gsl::span<const float>]’:
/opt/conda/include/gmock/gmock-matchers.h:2303:10:   required from here
/opt/conda/include/gmock/gmock-matchers.h:2312:48: error: no type named ‘const_iterator’ in ‘testing::internal::PointwiseMatcher<testing::internal::
FloatingEq2Matcher<float>, gsl::span<const float> >::Impl<const gsl::span<const float>&>::LhsStlContainer’ {aka ‘class gsl::span<const float>’}
```

cmake/external/onnxruntime_external_deps.cmake

@@ -46,8 +46,8 @@ if (onnxruntime_BUILD_UNIT_TESTS)
   FetchContent_Declare(
     googletest
     URL ${DEP_URL_googletest}
-    FIND_PACKAGE_ARGS NAMES GTest
     URL_HASH SHA1=${DEP_SHA1_googletest}
+    OVERRIDE_FIND_PACKAGE
   )
 endif()