ci: Install torchjd in editable mode

[ValerianRey]

• pytest-cov does not produce a code coverage report if the project is not installed in editable mode. So this fixes code coverage.
• The -e flag is also added for installations where no code coverage is computed, just in case.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 52 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.