Disable installing gtest when `cmake --install` #74

Job	Run time
test (ubuntu-gcc-werror, ubuntu, gcc, g++, -DCMAKE_CXX_FLAGS='-Werror=all -Werror=extra')	51s
test (ubuntu-gcc-aubsan, ubuntu, gcc, g++, -DCMAKE_CXX_FLAGS=-fsanitize=address -fsanitize=undefi...	1m 22s
test (ubuntu-gcc-tsan, ubuntu, gcc, g++, -DCMAKE_CXX_FLAGS=-fsanitize=thread)	1m 14s
test (ubuntu-gcc-static, ubuntu, gcc, g++)	1m 39s
test (ubuntu-gcc-dynamic, ubuntu, gcc, g++, -DBUILD_SHARED_LIBS=on)	59s
test (ubuntu-clang-static, ubuntu, clang, clang++)	1m 20s
test (ubuntu-clang-dynamic, ubuntu, clang, clang++, -DBUILD_SHARED_LIBS=on)	54s
test (ubuntu-gcc-static-cxx20, ubuntu, gcc, g++, -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQ...	1m 14s
test (ubuntu-gcc-static-cxx23, ubuntu, gcc, g++, -DCMAKE_CXX_STANDARD=23 -DCMAKE_CXX_STANDARD_REQ...	53s
test (ubuntu-gcc-static-cxx26, ubuntu, gcc, g++, -DCMAKE_CXX_STANDARD=26 -DCMAKE_CXX_STANDARD_REQ...	57s
test (ubuntu-clang-static-cxx20, ubuntu, clang, clang++, -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STAN...	1m 20s
test (ubuntu-clang-static-cxx23, ubuntu, clang, clang++, -DCMAKE_CXX_STANDARD=23 -DCMAKE_CXX_STAN...	1m 23s
test (ubuntu-clang-static-cxx26, ubuntu, clang, clang++, -DCMAKE_CXX_STANDARD=26 -DCMAKE_CXX_STAN...	1m 1s
	15m 7s

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