The az containerapp job start has a bug or it's not well written on the documentation on how to solve this issue.

However, there is a hack, that allows you to pass --env-vars and/or --command (if you need to change the args) to az containerapp job start.

You must include the --image on az containerapp job start and in this case it will accept the --env-vars.

This hack is described in this github issue:
https://github.com/Azure/azure-cli/issues/27023#issuecomment-1655749757