mapperLocal: actually leave loop on read from done channel

staticcheck points out that the break is otherwise an ineffective
break because it just ends the current case clause of the switch
it's in, which is true.

executor.go

@@ -5939,6 +5939,7 @@ func (e *executor) mapperLocal(ctx context.Context, shards []uint64, mapFn mapFu
 	ch := make(chan mapResponse, len(shards))
 
 	expected := 0
+	shardLoop:
 	for _, shard := range shards {
 		j := job{
 			shard:           shard,
@@ -5949,7 +5950,7 @@ func (e *executor) mapperLocal(ctx context.Context, shards []uint64, mapFn mapFu
 		}
 		select {
 		case <-done:
-			break
+			break shardLoop
 		case e.work <- j:
 			expected++
 		}