Restore old Evaluator switch, and put the new Evaluator behind a flag… #573

Job	Run time
Sjsonnet build for jvm on JDK 17	4m 21s
Sjsonnet build for jvm on JDK 21	4m 33s
Sjsonnet build for js on JDK 17	3m 4s
Sjsonnet build for js on JDK 21	3m 6s
Sjsonnet build for native on JDK 17	3m 34s
Sjsonnet build for native on JDK 21	3m 49s
	22m 27s

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