build.js: Stringify object args passed to tasks to avoid [Object object] in logs

[stevegolton]

At some point we added an object to the tsc task configuration in build.js but the logger that prints out the task info to the console didn't support object serialization - ending up with the following:

[6.947] 84/89	tsc --project ../../ui [object Object]
[20.886] 85/89	tsc --project ../../ui/src/service_worker [object Object]
[21.324] 86/89	tsc --project ../../ui --watch --preserveWatchOutput [object Object]

This patch simply runs args through JSON.stringify() in order to make them readable.

[github-actions]

🎨 Perfetto UI Builds

• e8cc214 (2026-04-03 13:05 UTC): https://storage.googleapis.com/perfetto-ci-artifacts/gh-23947091591-1-ui/ui/index.html