RunCMake/CXXModules/NoCXX20: Force older standard

Compile features ensure a compiler is aware of a standard version, but the one
actually used may be newer. The test relies on the standard chosen being
pre-C++20, so force C++17 explicitly.

This was exposed by a nightly bot that has a compiler defaulting to C++23. The
test would've broken anyway in a few years once GCC or Clang upped their
default.