Heroku pipeline - staging env variable carried into production

You cannot rebuild the slug, the main point of pipelines is to move the same slug between apps.

What you need to do is to fetch API_URL at runtime and not during build process. You can put all envs in one file, for example env.js

export const API_URL = process.env.API_URL;
export const OTHER_VAR = process.env.OTHER_VAR;

And then just import what you need in other files

import { API_URL, OTHER_VAR } from 'env.js';