ir3: Don't save/restore disasm string pointer

It's not in the key, so it randomly may or may not be present, and if it is present then we don't actually save/restore the contents, so we will save/restore random pointer values from the last run. Turnip already disables searching the shader cache when assembly is requested, but still wrote the final ir3_shader_variant which resulted in trying to save random stale pointers when saving off the executable if a subsequent compile hit that cache entry.

This fixes flakes in dEQP-VK.pipeline.pipeline_library.shader_module_identifier.pipeline_from_id.* for me.

Fixes: 56909868 ("turnip: implement VK_KHR_pipeline_executable_properties")