settings should be scoped to allow multiple plug-ins to run (e.g. scriptClasspath)

[Sciss]

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I run into multiple issues where settings are not isolated between plugins. For example, I want to build both a universal-zip and a debian installer. It is not possible to configure the scriptClasspath independently for these two plugins. Using scriptClasspath in Universal := ... has no effect, only setting it globally scriptClasspath := ... has, therefore influencing both universal and debian installer.