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ARROW-16507: [CI][C++] Use system gtest with mamba/conda

The gtest package for Windows provided by conda-forge doesn't provide `GTestConfig.cmake`.
See also: https://github.com/conda-forge/gtest-feedstock/blob/main/recipe/bld.bat

And `FindGTest.cmake` provided by CMake can't find `gtest_dll.dll` that is a shared
library version of GoogleTest.
See also: https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/FindGTest.cmake

It means that we can find only static version of GoogleTest on Windows with Conda
without a custom `FindGTestAlt.cmake`.

Shared library version `arrow_flight_testing` requires shared library version GoogleTest on
Windows because it defines `arrow::flight::FlightTest` that inherits `testing::Test`.
See also: https://github.com/apache/arrow/blob/master/cpp/src/arrow/flight/test_definitions.h

We must use the same library type for them on Windows.

To support `ARROW_FLIGHT=ON`/`ARROW_BUILD_SHARED=ON`/`ARROW_BUILD_STATIC=OFF`/
`ARROW_BUILD_TESTS=ON` with static version of GoogleTest, we need to build a static library not
shared library for  `arrow_flight_testing`.

Closes #13101 from assignUser/ARROW-16507-fix-gtest2

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Jacob Wujciak-Jens <jacob@wujciak.de>
Signed-off-by: Antoine Pitrou <antoine@python.org>