Allow transitions for builtin rules without allowlist

Only Blaze team members can write a built-in rule. Instead of having to modify
the allowlist every time we starlarkify a native transition, it's preferable to
always allow builtins to use Starlark transitions.

RELNOTES:none
PiperOrigin-RevId: 370772711

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkRuleClassFunctions.java

@@ -792,7 +792,8 @@ public void export(EventHandler handler, Label starlarkLabel, String ruleClassNa
       }
       // TODO(b/121385274): remove when we stop allowlisting starlark transitions
       if (hasStarlarkDefinedTransition) {
-        if (!hasFunctionTransitionAllowlist) {
+        if (!starlarkLabel.getRepository().getName().equals("@_builtins")
+            && !hasFunctionTransitionAllowlist) {
           errorf(
               handler,
               "Use of Starlark transition without allowlist attribute"