fix(terraform): call merge_environment_settings for prepared request

[dirgapeter]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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:

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

[stale]

Thanks for contributing to Checkov! We've automatically marked this issue as stale to keep our issues list tidy, because it has not had any activity for 6 months. It will be closed in 14 days if no further activity occurs. Commenting on this issue will remove the stale tag. If you want to talk through the issue or help us understand the priority and context, feel free to add a comment or join us in the Checkov slack channel at codifiedsecurity.slack.com
Thanks!

[stale]

Closing issue due to inactivity. If you feel this is in error, please re-open, or reach out to the community via slack: codifiedsecurity.slack.com Thanks!