provide test coverage for AWS temporary credential codepath

[Adam Sachs]

Is your feature request related to a specific problem?

In #4603, support was added for use of temporary AWS credentials in our AWS generate and scan code paths (thank you @tariqajyusuf for the contribution there!).

We don't yet have automated test coverage for this particular feature.

Describe the solution you'd like

We should add automated test coverage for the feature! Shouldn't be too bad, we'll just need our test fixture to (dynamically) retrieve some temporary credentials using our permanent credentials stored as GH secrets, and then pass those along to the generate/scan APIs 👍

Describe alternatives you've considered, if any

Stay without automated test coverage. It's a relatively narrow feature, so there's frankly a pretty small chance of uncaught regressions without automated test coverage. But it's always better to have automated test coverage where we can!

Additional context

Initial update was made as part of an open source contribution, which makes it difficult (impossible?) to get automated test coverage in this case, since test coverage here requires access to external AWS resources that rely on our GH repo secrets.