ci: Centralised Poetry install

[khvn26]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

In this PR, we:

• Make sure the dev environment is available in the API Run Makefile Target workflow.
• Rely on make install-poetry target in CI.
• Correctly name the run-makefile-target job.
• Remove obsolete if: steps.cached-poetry-dependencies.outputs.cache-hit != 'true' directives.

How did you test this code?

This is a CI change that will be fully tested once it's merged.

[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-6039	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-6039	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-6039	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-6039	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-6039	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-6039	Finished ✅	Results ✅

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

3 Skipped Deployments

Project	Deployment	Preview	Comments	Updated (UTC)
docs	Ignored	Preview		Sep 9, 2025 2:32pm
flagsmith-frontend-preview	Ignored			Sep 9, 2025 2:32pm
flagsmith-frontend-staging	Ignored	Preview		Sep 9, 2025 2:32pm

[emyller]

🔪

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.99%. Comparing base (79b7cbf) to head (95b7108).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6039   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files        1275     1275           
  Lines       44892    44892           
=======================================
  Hits        43993    43993           
  Misses        899      899           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.