CVC4: update CVC4 to v1.8.1-X to remove buggy finalizers (again)

[kfriedberger]

Default CVC4 versions trigger random segmentation faults due to issues with Java finalizers. This PR switches to a custom build that disables them to ensure stability.

This introduces memory leaks for solver-related objects (Terms, Types, SmtEngine, ExprManager). This behavior is consistent with our last 6 years of operation and is deemed an acceptable trade-off for preventing JVM crashes.

Build source: kfriedberger/CVC4@1f9dbdc

PS: If someone wants to play around in Java, we could try to close at least SmtEngine and ExprManager once no longer needed. All objects provide the operation for delete().

[daniel-raffler]

Thanks for looking into it! It's already looking a lot better. I think you can just remove the @After in CVC4NativeAPITest to get rid of the remaining crashes:

  @After
  public void freeEnvironment() {
    smtEngine.delete();
    exprMgr.delete();
  }