Set PATH in build_release.py when calling tox

On Windows, we need to have the proper Python folder in PATH when using PyQt5,
so it can load python3.dll properly.

scripts/dev/build_release.py

@@ -62,6 +62,7 @@ def call_tox(toxenv, *args, python=sys.executable):
     """
     env = os.environ.copy()
     env['PYTHON'] = python
+    env['PATH'] = os.environ['PATH'] + os.pathsep + os.path.dirname(python)
     subprocess.check_call(
         [sys.executable, '-m', 'tox', '-v', '-e', toxenv] + list(args),
         env=env)