isNonProdBuildEnvironment check should print more precise info

[xixixao]

When isNonProdBuildEnvironment is used in the CLI and it fails the build, it should say specifically I detected VERCEL because VERCEL env var is truthy, but the VERCEL_ENV env var is not set to "production" etc.

[nipunn1313]

Yep agreed! The check in deploy.ts could give a better error message. PR welcome!