lint: fix custom mypy cache dir setting

test/lint/lint-python.py

@@ -9,14 +9,17 @@
 """
 
 import os
+from pathlib import Path
 import subprocess
 import sys
 
 from importlib.metadata import metadata, PackageNotFoundError
 
+# Customize mypy cache dir via environment variable
+cache_dir = Path(__file__).parent.parent / ".mypy_cache"
+os.environ["MYPY_CACHE_DIR"] = str(cache_dir)
 
 DEPS = ['flake8', 'lief', 'mypy', 'pyzmq']
-MYPY_CACHE_DIR = f"{os.getenv('BASE_ROOT_DIR', '')}/test/.mypy_cache"
 
 # All .py files, except those in src/ (to exclude subtrees there)
 FLAKE_FILES_ARGS = ['git', 'ls-files', '*.py', ':!:src/*.py']