Fix installation of iOS targets (#12506)

Since cmTarget::ComputeOutputDir results can be used in CMake code of script
cmake_install.cmake and in Xcode internals, string ${EFFECTIVE_PLATFORM_NAME}
should be used instead of $(EFFECTIVE_PLATFORM_NAME) because it works for both.

Value of CMAKE_CFG_INTDIR can't be used in BUILD_TYPE argument of install
command since it contains $(EFFECTIVE_PLATFORM_NAME) (e.g. equals to
`Release-iphoneos`, `Debug-iphoneos`, etc.).