Odoo.sh does not provide any way to set environment variables. There are two existing environment variables:

ODOO_STAGE (prod vs staging vs dev)

ODOO_VERSION (16.0 vs 17.0 vs 18.0)

See the FAQ for details: https://www.odoo.sh/faq?q=ODOO_STAGE

If you are using libraries or third party apps that depend on a given environment variable we recommend the system parameter approach.