CCM-16484: Improve lambda_env_vars output

[jamesthompson26-nhs]

Description

Simplify the lambda_env_vars output to make it less defensive.

Context

Though it looks more safe and defensive, where the module calls insert something into the variable that cannot be resolved at plan time (Skynet hit this with s3bucket.this.id) it can cause the plan to fail.

Deployed to sbx env in template management, using this branch to modify the source of 33 lambdas in the backend-api module:

#173
https://github.com/NHSDigital/nhs-notify-internal/actions/runs/24185496908

Deploys successfully and all tests pass/

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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