BAU: guardrail to ensure pentester email parity

[whi-tw]

What problem does this pull request solve?

As we maintail the pentester email addresses in two places, we need to
nsure they are kept in sync. This guardrail checks that the email
addresses match in both locations and that they are only set in staging.

This is similar to the guardrail we have for the CIDR blocks.

Things to consider when reviewing

• Ensure that you consider the wider context.
• Does it work when run on your machine?
• Is it clear what the code is doing?
• Do the commit messages explain why the changes were made?
• Are there all the unit tests needed?
• Has all relevant documentation been updated?

Reminders

If you've made changes to the deployer role (files in modules/deployer-access):

• Remember to run make <environment> forms/account apply on the relevant environments (dev, staging and/or prod)
• Check the #govuk-forms-deployment-notifications Slack channel to ensure the apply-forms-terraform-<environment> pipelines have run successfully