cxxmodules: Honor CMAKE_CXX_SCAN_FOR_MODULES in try_compile
Under CMP0155's NEW behavior, projects that explicitly enable C++20
support with set(CMAKE_CXX_STANDARD 20)
may also explicitly
disable scanning with set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
.
We already propagate CMAKE_CXX_STANDARD
into try_compile
test
projects, so propagate CMAKE_CXX_SCAN_FOR_MODULES
too.
Fixes: #25313 (closed)
Backport: release