thanks!
It is a known shortcoming that the
netlify.tomlfile environment variables are not usable in functions. You must set them in our UI in the Build & Deploy settings page, and then to use them, you must ALSO change your function in some way to redeploy it.
Just to be clear, this means that there is no way to have specific Env Var values for your branch deploy functions. Because all functions (regardless of what branch they are associated with) will use the Env Vars that were set in the UI. There is no workaround that solves this issue (aside from hosting a separate site and using it purely as a staging environment as suggested by @pgarciacamou)