The variable here gets added to the nir_shader::system_values list
rather than the nir_shader::inputs list, so it should have the
nir_var_system_value variable mode instead of nir_var_shader_in.

Not doing this will have horrible consequences when rebasing on
upstream, because all of these variables will be stored in the same list
there. This means the variable-mode is the only detail left to let us
know what this is.