I have an envvar I need to set, and it is exactly https://${VERCEL_URL}. Printing process.env.VERCEL_URL shows that that is fine, but printing process.env.MYVAR gives: https://${VERCEL_URL}... namely the envvar has not resolved. How can I use system variables in my environment variables? such as VERCEL_URL, VERCEL_PROJECT_PRODUCTION_URL etc.