ci: make infra-only step failures non-fatal before Required Checks becomes required #5860

What is the problem the feature request solves?

#5842 adds a Required Checks aggregator job, and #5838 will make it a required status check on main and turn on a merge queue. Once that happens, any red job in ci.yml evicts the PR from the queue.

The first two CI runs on #5842 each had exactly one failure, and neither was a test failure:

1. PR Build (Linux) / Spark 4.1, JDK 17 [expressions] ran 1378 tests with 0 failures, then the Upload test results step (.github/actions/java-test/action.yaml, actions/upload-artifact@v7) failed with Failed to FinalizeArtifact: Received non-retryable error: Failed request: (403) Forbidden: Error from intermediary. The identical step in the sibling Spark 4.2 leg of the same run succeeded.
2. PR Build (Linux) / Lint Scala (syntactic) failed in cs launch scalafix:0.14.6 (.github/workflows/pr_build_linux.yml) with coursier.error.FetchError$DownloadingArtifacts ... Caught java.net.SocketException (Connection reset) fetching a guava jar from Maven Central.

Neither step affects the test verdict. Today they cost a manual re-run. With a merge queue they cost a queue slot and a full re-run of the pipeline.

Describe the potential solution

• Mark the Upload test results step in .github/actions/java-test/action.yaml continue-on-error: true. The surefire reports are a debugging aid, not part of the verdict.
• Wrap the coursier launch in Lint Scala (syntactic) in a short retry loop, or pre-warm the coursier cache with actions/cache, so a single transient Maven Central reset does not fail the job.
• Audit the other reusable workflows for steps of the same shape (artifact upload, tool download) that run after the verdict is already known.

This should land before the #5838 follow-up that names Required Checks in .asf.yaml.

Additional context

Failing runs: https://github.com/apache/datafusion-comet/actions/runs/34557062947/job/103137767294 and https://github.com/apache/datafusion-comet/actions/runs/34600387981/job/103266393475