Only import importlib_metadata when needed

Similar to #395, this helps with bootstrap issues with python < 3.8.

src/build/env.py

@@ -18,11 +18,6 @@
 import build
 
 
-if sys.version_info < (3, 8):
-    import importlib_metadata as metadata
-else:
-    from importlib import metadata
-
 try:
     import virtualenv
 except ModuleNotFoundError:
@@ -255,6 +250,11 @@ def _create_isolated_env_venv(path: str) -> Tuple[str, str]:
 
     import packaging.version
 
+    if sys.version_info < (3, 8):
+        import importlib_metadata as metadata
+    else:
+        from importlib import metadata
+
     venv.EnvBuilder(with_pip=True, symlinks=_fs_supports_symlink()).create(path)
     executable, script_dir, purelib = _find_executable_and_scripts(path)