CI: save runner disk — no pip cache, no separate install job

The git-run-ia2 runner is out of disk and `pip install -e .` fails.
- PIP_NO_CACHE_DIR instead of a pip cache in the project dir, and no
  GitLab cache: the wheels were kept twice and stored between jobs
- drop install_project: run_pylint and pages already run
  `pip install -e .`, so a broken install still fails the pipeline

The real fix is freeing disk space on the runner.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>