Fix issue where dependency caching was not done properly in the backend

[Dref360]

Description:

Fix issue where dependency caching was not done properly in the backend

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge
it.

• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• FRONTEND TYPES. Regenerate the front-ent types if you played with types and routes.
  Run cd webapp && yarn types while the back-end is running.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[JosephMarinier]

Can you explain how does that fix the issue?

[Dref360]

This line would install dependencies outside of poetry, ie in the system python folder.
So the cache step didn't trigger as ~/.cache/poetry was always empty. Now it trigger correctly based on the MD5 of poetry.lock

[Dref360]

I reran the CI and now the step that took ~8min took 8 seconds!