perf: #3616266 Reuse the installed build cache in the parallel test jobs

Speeds up the test stage by reusing the installed build in the 15 parallel test jobs instead of rebuilding it in each. Issue: #3616266.

Measured baseline (a recent green 11.0.x test job): ~75s composer install + ~66s yarn install + ~33s playwright install before cucumber even starts; the DB restore itself is only ~1s.

• The Install Varbase job now also builds node_modules + browsers and leaves vendor, node_modules, the web docroot (incl. the drupal-libraries-sync output and installed files) and the Playwright browsers in the shared build cache.
• The test jobs pull that cache (pull-only) and skip composer/yarn/playwright — they set up only the OS libs, Apache and the Node runtime, restore the DB dump, and run.

This is an experiment branch to compare real pipeline durations before applying to 11.0.x / 10.1.x / 9.2.x. If the cache transfer offsets the win, a single-tar-artifact variant is the fallback.

Checkpoints:

• File an issue
• Addition/Change/Update/Fix
• Testing to ensure no regression
• Automated unit testing coverage
• Automated functional testing coverage
• UX/UI designer responsibilities
• Readability
• Accessibility
• Performance
• Security
• Developer Documentation
• User Guide Documentation
• Reviewed by human
• Code review by maintainers
• Full testing and approval
• Credit contributors
• Review with the product owner
• Release notes snippet
• Release