Can I set a project variable to run different build scripts among several environments?

Only project level variables are available during build time, so you will not be able to create a variable that is different in each environment that will be accessible during builds.

You can create a project level variable using the CLI:

$ upsun variable:create --level project --name NAME --value 'some-value' --json false --sensitive false --prefix 'env:' --enabled true --visible-build true --visible-runtime false --project wiqte7at4yg22

Each environment in your project will need to be redeployed in order for the changes to take affect by using the command for each environment.

$ upsun redeploy <environment name>