Check if cmd is list before accessing elements

Author: alex-lisi

platformio/package/manager/base.py

@@ -334,7 +334,7 @@ def call_pkg_script(self, pkg, event):
             cmd = [cmd]
         os.environ["PIO_PYTHON_EXE"] = get_pythonexe_path()
         with fs.cd(pkg.path):
-            if os.path.isfile(cmd[0]) and cmd[0].endswith(".py"):
+            if isinstance(cmd, list) and os.path.isfile(cmd[0]) and cmd[0].endswith(".py"):
                 python_exe = os.environ["PIO_PYTHON_EXE"]
                 # Quote the path if it contains spaces (Windows compatibility)
                 if " " in python_exe: