Skylark: Fix SkylarkNestedSet.toString

--
MOS_MIGRATED_REVID=85874312
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
index 17fc55f..a1b1e26 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/SkylarkNestedSet.java
@@ -190,4 +190,9 @@
   public Class<?> getGenericType() {
     return genericType;
   }
+
+  @Override
+  public String toString() {
+    return "set(" + set.toCollection() + ")";
+  }
 }