chore: cleaner exception tracebacks

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

cibuildwheel/linux.py

@@ -367,7 +367,8 @@ def build(options: Options, tmp_path: Path) -> None:  # pylint: disable=unused-a
     cwd = Path.cwd()
     abs_package_dir = options.globals.package_dir.resolve()
     if cwd != abs_package_dir and cwd not in abs_package_dir.parents:
-        raise Exception("package_dir must be inside the working directory")
+        msg = "package_dir must be inside the working directory"
+        raise Exception(msg)
 
     container_project_path = PurePosixPath("/project")
     container_package_dir = container_project_path / abs_package_dir.relative_to(cwd)