You're forcing a core context, which in turn disables things like
glGetString(GL_EXTENSIONS); you have to use glGetStringi now. However cemu
isn't ready for a core context.

Try MESA_GL_VERSION_OVERRIDE=4.0COMPAT -- that should have the desired effect.
Note that tons of stuff doesn't work in such high-versioned compat contexts.
Any time you're setting any overrides, it's your problem rather than a mesa
problem...