Improve secret interpolation string in enp deploy workflow

[coreycarvalho]

User Story - Business Need

The deploy workflow (https://github.com/department-of-veterans-affairs/va-enp-api/blob/main/.github/workflows/deploy.yml#L56) uses logic that will continue to grow and look ugly as we add more environments.

We should fix this to be more readable and general purpose.

• Ticket is understood, and QA has been contacted (if the ticket has a QA label).

User Story(ies)

As a VA Notify developer
I want to fix the secret interpolation string in enp deploy workflow
So that adding new environments is less burdensome.

Additional Info and Resources

Acceptance Criteria

• When I kick off the deploy workflow, the correct secrets are used in the ENP task definition
• The above is done without using boolean logic in the workflow
• This work is added to the sprint review slide deck (key win bullet point and demo slide)

QA Considerations

Potential Dependencies