vinyl: store pointer to vy_run_env in vy_run

It's really annoying to pass vy_run_env along with vy_run or vy_slice
every time we want to read a run. Let's store a pointer to vy_run_env
immediately in vy_run. This is a widely accepted practice throughout
vinyl - we already do this in case of vy_index, vy_cache, and vy_mem.