unable to read contents of the file C:\petstore\target\gen\$MGW_HOME\conf\micro-gw.conf

With the following block in deployment_config.toml, I got error as titled.
[kubernetes.kubernetesConfigMap]
    enable = true
    ballerinaConf = '${MGW_HOME}/conf/micro-gw.conf'

I change the  "${MGW_HOME}" to "$MGW_HOME" or "%MGW_HOME%", but the environment variable  MGW_HOME can not be extended.

It seems  "ballerinaConf" can only use full path or path relative to  target\gen .
 