test: make verify_clean_boot respect return code by environment

[aciba90]

Proposed Commit Message

test: make verify_clean_boot respect return code by environment

GH checks should now pass when run out of a pr event (as the post merge
event) while still work on prs and Jenkins.

Additional Context

https://github.com/canonical/cloud-init/commit/a9c6b1407714b70c11b72b1c32489dacf4bc4000 https://github.com/canonical/cloud-init/actions/runs/11073687167/job/30770781093

Test Steps

export CLOUD_INIT_CLOUD_INIT_SOURCE=IN_PLACE
export CLOUD_INIT_PLATFORM=lxd_container
export CLOUD_INIT_OS_IMAGE=focal

tox -e integration-tests -- -vvv \
  tests/integration_tests/modules/test_combined.py::TestCombined::test_deprecated_message \
  tests/integration_tests/modules/test_users_groups.py::TestUsersGroups::test_user_root_in_secret \
  tests/integration_tests/modules/test_users_groups.py::TestUsersGroups::test_nopassword_unlock_warnings

Merge type

• [x] Squash merge using "Proposed Commit Message"
• [ ] Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#)