Allow for a different app_build_command to be run between staging and production envs

Loving static web apps so far!

Right now we have the ability to set different env vars between staging and prod azure functions but no way to define custom envs for the front end app between stage and prod. This could be solved by allowing users so specify a custom app_build_command in their Github Actions workflow file that would be run when deploying to stage.

For example, a user could specify that their CRA React app should run `npm build` for prod but `npm build:staging` for staging or something along those lines.