Flink 2.1: Fix forward-writer chaining regression in DynamicIcebergSink

[drexler-sky]

Flink CI failed in the following PRs:
#16006
#16023
#16024

TestDynamicIcebergSink > testNoShuffleTopology() FAILED
    org.opentest4j.AssertionFailedError: 
    Expecting value to be true but was false
        at app//org.apache.iceberg.flink.sink.dynamic.TestDynamicIcebergSink.testNoShuffleTopology(TestDynamicIcebergSink.java:330)

This PR fixes that failure.

[manuzhang]

@drexler-sky Have you rebased on latest main branch? This fix still fails in my local test.

[huaxingao]

cc @pvary @stevenzwu @sachinnn99 @sqd

[sqd]

LGTM, thank you. @manuzhang I cherrypicked this onto main, and verified that this fixed the CI failure. Am I missing something? I ran just the failing test which is ./gradlew :iceberg-flink:iceberg-flink-2.1:test --tests org.apache.iceberg.flink.sink.dynamic.TestDynamicIcebergSink

[nssalian]

Thanks for the prompt fix @drexler-sky. CI run for your PR was clean and I ran this locally to check too after adding your patch:

./gradlew :iceberg-flink:iceberg-flink-2.1:test --tests org.apache.iceberg.flink.sink.dynamic.TestDynamicIcebergSink

and

./gradlew -DsparkVersions= -DkafkaVersions= -DflinkVersions=2.1 :iceberg-flink:iceberg-flink-2.1:check :iceberg-flink:iceberg-flink-runtime-2.1:check -Pquick=true -x javadoc -DtestParallelism=auto

@pvary @stevenzwu PTAL

[huaxingao]

Thanks @drexler-sky for the PR! Thanks everyone for the review!