Java 18 and 19 SecurityManager handling

[srdo-humio]

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rules_scala doesn't work out of the box with Java 18 or 19, since Worker.java makes use of a SecurityManager, which is not allowed by default as of Java 18. See https://openjdk.org/jeps/411.

Since there is no replacement API to trap System.exit yet, the solution is to pass -Djava.security.manager=allow to the JVM running the worker.

It is possible to do this explicitly in my own project by passing the flag via scalac_jvm_flags in my Scala toolchain, but I was wondering if we should do something in rules_scala to make this more obvious to new users?

Sadly I don't think it will work to pass the flag always, since older JDKs don't understand the flag.

Adding a bit to the README about this might be good enough. Alternatively rules_scala could pass that flag to the worker automatically if the JVM it will use is 18+ (I'm not sure how best to detect this in Starlark, parsing java --version maybe?).