`myst exec` should fall back to the executable path and parameters in config.json with `--app-config-path`

[jxyang]

When running in debug mode with --app-config-path and without the path to the executable and the parameter as command line options, the myst command should be smart enough to check the config file for a possible specification of the executable path and parameters.

[bodzhang]

With myst-exec, if --app-config-path is provided, the executable path in config.json is always used, ignoring the executable path in the command line. But if the command line does not include any executable path, myst-exec fails.

[bodzhang]

myst exec-linux ignores --app-config-path. This behavior is inconsistent with myst exec-sgx.