add in scala-update to contrib

[ckipp01]

Relates to https://github.com/kitlangton/scala-update/issues/5.

@alexarchambault how does it work if all the platforms aren't listed? For example scala-update doesn't publish an image for windows, so does it just fall back to the JVM launcher if the user is on a platform that a prebuilt binary doesn't exist for?

cc @kitlangton

[kitlangton]

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