Use shortName property for jar

[sghill]

In order to improve predictability
As a gradle-jpi-plugin user
I want the jenkinsPlugin.shortName to also influence the base name of the jar

Today the shortName controls the output of the jpi, but the jar is always named after the project.

// build.gradle
jenkinsPlugin {
    coreVersion = '2.222.3'
    shortName = 'sun'
}
version = '1.0.0'

// settings.gradle
rootProject.name = 'moon'

$ ./gradlew jpi && ls -la build/libs
    sun.jpi
    moon-1.0.0.jar