Fix Gradle Job-Token example (add System.getenv)

What does this MR do?

Fix small issue in the example of using Job-Token in Gradle:

credentials(HttpHeaderCredentials) {
    name = 'Job-Token'
    // before
    value = '${CI_JOB_TOKEN}' // variable from gradle context, does not exist
    // after
    value = System.getenv()['CI_JOB_TOKEN'] // variable from environment which is what required here
}

Conformity

• Documentation (if required)