feat: optional `TURBO_SITE` environment variable at compilation time for development (#9660)
### Description
To make it easier to test out the links we supply in our error messages,
this PR lets us supply a base URL of our own choosing so you can point
errors at places like `http://localhost:3000` or a preview build of
`turbo.build`.
### Testing Instructions
You can try this out yourself with:
```bash
TURBO_SITE="http://localhost:3000" cargo build
```