[cxxmodules] Fix the test-periodic-build if -Dcxxmodules=On.

The test should reuse the prebuilt modules in the module cache. However,
due to a mismatch between the module-related flags exported by FindROOT.
There is a mismatch between the -D passed by cmake.

This patch make sure that the build arguments match more closely the build
setup of ROOT. Thus we can reuse the already built module files.