feat: enable setting environment variables on the function

In most cases, it is more convenient to set non-secret information via environment variables. Expose this functionality.

This MR also contains a few small fixes:

• The syntax in outputs.tf was incorrect if a custom service account was supplied. In this case the google_service_account.script tuple would be empty and google_service_account.script[0] would error. Move to using the ?! operator which shortcuts its arguments and so never evaluates the troublesome expression if a custom service account is provided.
• The pre-commit hook markdownlint-cli2 is currently failing to initialise itself and download the nodejs runtime. Comment it out for the moment.