_service/pypi.py: assign pip_cache_dir outside of if statement

Assignment expressions are only supported from Python 3.8 and upwards.
Because the if statement consists out of early returns, we can create
another if statement and assign the pip cache directory before the if
statement and after the check for a custom directory has happened.

pip_audit/_service/pypi.py

@@ -111,9 +111,9 @@ def _get_pip_cache() -> str:
 def _get_cache_dir(custom_cache_dir: Optional[Path]) -> str:
     if custom_cache_dir is not None:
         return str(custom_cache_dir)
-    elif (
-        pip_cache_dir := _get_pip_cache() if _PIP_VERSION >= _MINIMUM_PIP_VERSION else None
-    ) is not None:  # pragma: no cover
+
+    pip_cache_dir = _get_pip_cache() if _PIP_VERSION >= _MINIMUM_PIP_VERSION else None
+    if pip_cache_dir is not None:  # pragma: no cover
         return pip_cache_dir
     else:
         fallback_path = os.path.join(Path.home(), ".pip-audit-cache")