Export GOOGLE_CHROME_BIN from the runner's preinstalled Chrome

[wesrich]

Summary

• Some Rails apps shell out to Chrome directly (e.g. server-side PDF generation via system("... $GOOGLE_CHROME_BIN ...")) rather than only using it through a browser-driver gem, and previously had to carry their own Setup Chrome step in their own ci.yml to get that path.
• Blacksmith's ubuntu runner images already ship Chrome, so this just resolves it with which in a background step alongside the other early setup steps, and exports GOOGLE_CHROME_BIN for the rest of the job.

Test plan

• private repo is temporarily pointing its CI at this branch to confirm PDF-generation specs pass with the exported path