Ignore API change in kotlinx-coroutines-core in train build

Ignored change in interface ReceiveChannel interface is:
-  public abstract fun receiveOrClosed (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
+  public abstract fun receiveOrClosed-ZYPwvRU (Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
This change is planned in 1.4.0 compiler.
diff --git a/build.gradle b/build.gradle
index 4dc0481..a758393 100644
--- a/build.gradle
+++ b/build.gradle
@@ -132,6 +132,7 @@
     if (build_snapshot_train) {
         ignoredProjects.remove("site")
         ignoredProjects.remove("example-frontend-js")
+        ignoredProjects.add("kotlinx-coroutines-core")
     }
     ignoredPackages += "kotlinx.coroutines.internal"
 }