feat: Ensure RVM caches deterministic builtins

Mirror interpreter implementation:
- use builtins::must_cache to determine whether builtin must be cached.
- reuse cached value when applicable
- clear the VM’s builtin cache whenever execution state resets to avoid leaking values across runs
- add a YAML regression for rand.intn set comprehensions and re-enable the rand cases in the OPA test suite

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/opa.rs

@@ -36,7 +36,6 @@ const OPA_TODO_FOLDERS: &[&str] = &[
     "partialdocconstants",
     "partialobjectdoc",
     "planner-ir",
-    "rand",
     "refheads",
     "replacen",
     "semverisvalid",