Fix yarn install in CI now that Varbase Project 11.0.x-dev ships no yarn.lock

Problem/Motivation

The "Automated Functional Testing" CI job fails on 1.0.x at yarn install:

Usage Error: The nearest package directory (/builds/project/varbase_news_base/.site) doesn't seem to be part of the project declared in /builds/project/varbase_news_base.

Varbase Project 11.0.x-dev no longer ships a yarn.lock (Back to DEV in #3629850). Without one, Yarn takes this recipe's own package.json as the project root.

Steps to reproduce

Run the 1.0.x pipeline, see job 12789350.

Proposed resolution

Create an empty yarn.lock in .site before yarn install, as Yarn suggests, so the test site is its own Yarn project.

Remaining tasks

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

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A