Fix Build and push complement image CI job not having Poetry for complement.sh

[MadLittleMods]

Fix Build and push complement image CI job not having Poetry for complement.sh

❌ Build and push complement image, https://github.com/element-hq/synapse/actions/runs/23176317296/job/67339146082

scripts-dev/complement.sh: line 227: poetry: command not found

Follow-up to #19523

This regressed in #19476

Testing strategy

1. Visit https://github.com/element-hq/synapse/actions/workflows/push_complement_image.yml
2. Run workflow:
   • Use workflow from: madlittlemods/fix-complement-push-image-ci-job-poetry
   • Branch: develop
3. Wait for CI to run and pass ✅

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[MadLittleMods]

CI is actually passing now ✅ : https://github.com/element-hq/synapse/actions/runs/23176910722/job/67341095251 (using the testing strategy from the PR description)

[sandhose]

Fine by me, modulo the poetry version which just changed

[MadLittleMods]

Thanks for the review @sandhose 🦪