[GOBBLIN-2144] Prevent GenerateWorkUnitsImpl from inadvertently cleaning up temp/staging dirs

[phet]

Dear Gobblin maintainers,

Please accept this PR. I understand that it will not be reviewed until I have checked off all the steps below!

JIRA

• My PR addresses the following Gobblin JIRA issues and references them in the PR title. For example, "[GOBBLIN-XXX] My Gobblin PR"
  • https://issues.apache.org/jira/browse/GOBBLIN-2144

Description

• Here are some details about my PR, including screenshots (if applicable):

Whereas the AbstractJobLauncher, used by Gobblin-on-MR, runs the entire job end-to-end, the Gobblin-on-Temporal GenerateWorkUnitsImpl activity solely generates WUs w/o actually processing them. Thus it's far too early to clean anything up in the temp/staging dirs (e.g. task-staging and task-output, used by writers and the commit step).

Even worse, depending upon config, doing cleanup nonetheless could destabilize other job executions, when those share this same temp/staging area. Such cross-job "action-at-a-distance" is unpredictable and especially challenging even to diagnose.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

./gradlew build successful

Commits

• My commits all reference JIRA issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

[phet]

NBD, just noticed this to be missing