fix for #13760 - running script compiled jar no longer restricted to java.base module

[philwalk]

This resolves the problem described in #13760.

The essence of the problem: running a script with the -save option will throw a ClassNotFoundException, but only when running from the compiled jar, and only if the script references modules other than java.base . The jar is fine when run via java -jar <jarfile>, the issue only occurs when scala launches the jar.

The workaround for the problem has been to append the -nosave option to the hashbang line.

The cause of the problem is that bootClassLoader in ScalaClassLoader is set to null when running in jdk9+.
The following command line illustrates the problem code at ScalaClassLoader line 70:

$ /opt/scala3/bin/scala -e 'import java.lang.invoke.*; MethodHandles.lookup().findStatic(classOf[ClassLoader], "getPlatformClassLoader", MethodType.methodType(classOf[ClassLoader])).invoke().asInstanceOf[ClassLoader]'

```scala
$ /opt/scala3/bin/scala -e 'import java.lang.invoke.*; MethodHandles.lookup().findStatic(classOf[ClassLoader], "getPlatformClassLoader", MethodType.methodType(classOf[ClassLoader])).invoke().asInstan
ceOf[ClassLoader]'
Exception in thread "main" java.lang.invoke.WrongMethodTypeException: cannot convert MethodHandle()ClassLoader to (Object[])Object
        at java.base/java.lang.invoke.MethodHandle.asTypeUncached(MethodHandle.java:861)
        at java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:847)
        at java.base/java.lang.invoke.MethodHandleImpl$WrappedMember.asTypeUncached(MethodHandleImpl.java:1371)
        at java.base/java.lang.invoke.MethodHandle.asType(MethodHandle.java:847)
        at java.base/java.lang.invoke.Invokers.checkGenericType(Invokers.java:495)
        at compileFromString$minus08906c21$minusee54$minus45f1$minusb470$minusfe08bfbca561$package$.main(compileFromString-08906c21-ee54-45f1-b470-fe08bfbca561.scala:2)
        at main.main(compileFromString-08906c21-ee54-45f1-b470-fe08bfbca561.scala:1)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:566)
        at dotty.tools.scripting.StringDriver.compileAndRun(StringDriver.scala:36)
        at dotty.tools.MainGenericRunner$.run$1(MainGenericRunner.scala:257)
        at dotty.tools.MainGenericRunner$.main(MainGenericRunner.scala:267)
        at dotty.tools.MainGenericRunner.main(MainGenericRunner.scala)
```

An explanation of why this code no longer works for jdk9+ is described here: Regression in behavior of a null parent ClassLoader

The proposed fix is based on the fix to #11658.