Speed up the test stage: reuse the installed build in the parallel test jobs instead of rebuilding

Problem/Motivation

Following with #3612699: Reduce CI usage

The functional testing pipeline already installs Varbase once in a pre-test job and restores the database in each of the 15 parallel test jobs (the DB restore is ~1s). But every test job still re-runs composer install (~75s), yarn install (~66s) and playwright install (~33s) — about 174s of build repeated 15 times, while only the site data actually needs to arrive.

Steps to reproduce

Read a recent pipeline's test job trace: the first ~200s is composer + yarn + playwright before cucumber starts; the DB restore itself is 1s.

Proposed resolution

• Have the Install Varbase job populate the built trees (vendor, node_modules, the web docroot incl. the drupal-libraries-sync output) and the Playwright browsers into the shared build cache.
• The 15 parallel test jobs pull that cache (pull-only) and skip composer/yarn/playwright entirely — they set up only the OS libs, Apache and the Node runtime, restore the DB dump (1s), and run.
• Measure the real pipeline durations against the current one; if the cache transfer offsets the win, compare a single-tar-artifact variant and keep the faster.

Remaining tasks

• ✅ 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

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• The parallel test jobs reuse the installed build from cache instead of rebuilding, cutting pipeline time.