Unfortuantely different versions of DXC accept or fail differently
in some cases. We don't care too much about validating the DXC
behavior itself, but it's useful that all the CI jobs use the
same version so that we don't have to complicate the shader runner
language.
The macOS version is currently bound to be pretty recent because
otherwise libdxil.dylib is not provided. So I'm updating the
Linux and Windows version as well.
I don't expect it should be particularly hard for other maintainers
to keep up with the DXC updates, since it just amounts to
downloading a ZIP file and extracting two libraries.
