nat-gateways module: Avoid conversion to set to fix ordering issue

[psibi]

• [x] Update the changelog
• [ ] Make sure that modules and files are documented. This can be done inside the module and files.
• [ ] Make sure that new modules directories contain a basic README.md file.
• [ ] Make sure that the module is added to tests/main.tf
• [ ] Make sure that the linting passes on CI.
• [ ] Make sure that there is an up to date example for your code:
  • For new modules this would entail example code for how to use the module or some explanation in the module readme.
  • For new examples please provide a README explaining how to run the example. It's also ideal to provide a basic makefile to use the example as well.
• [ ] Make sure that there is a manual CI trigger that can test the deployment.

[ketzacoatl]

Confirmed in private tracker.