pip 20.0.2 does not support relative paths for cache dir

pip 20.0.2 does not support relative paths for cache dir: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). Using normalized path should be backward compatible with older versions of pip as well, so it should be safe to do path normalization on pip-tools side.