See also the discussion in https://github.com/bitcoin/bitcoin/pull/24794/files#r844848504 . However, the variable is completely unused:

$ git grep MYPY_CACHE_DIR
test/lint/lint-python.py:MYPY_CACHE_DIR = f"{os.getenv('BASE_ROOT_DIR', '')}/test/.mypy_cache"

Not sure if it was ever needed. If not, it should be removed. cc @fjahr