Backport of drivers: update ordering of events in StartTask to fix executor leak into release/1.9.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #24495 to be assessed for backporting due to the inclusion of the label backport/1.9.x.

The below text is copied from the body of the original PR.

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Description

If an error occurs before a task is started, but after this executor process is created, the executor must be explicity stopped. This change updates the logic in StartTask so launching the task happens immediately after creating the executor.

Testing & Reproduction steps

Run a job with a configuration that would fail after the executor process has started, but before the task is launched. For example, an exec task with cap_add = ["net_raw"]. After the job fails, the executor process will still be running.

Links

Fixes GH #11958

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

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Overview of commits

• 11ae64a