fix(terraform): call merge_environment_settings for prepared request

[dirgapeter]

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Description

Call merge_environment_settings when prepared request flow is used, as described in https://requests.readthedocs.io/en/latest/user/advanced

Fixes #7276

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my feature, policy, or fix is effective and works
• [x] New and existing tests pass locally with my changes