DEVPROD-11883: ignore Parameter Store feature flag for GitHub app keys

[Kimchelly]

DEVPROD-11883

Description

Follow-up to #8592 - as part of switching to Parameter Store as the only storage for GitHub app private keys, this will start ignoring the temporary project-level feature flag. Instead, projects will always use Parameter Store for GitHub app private keys without needing me to enable the project-level flag.

Along with this, I noticed that the Spruce e2e tests were failing because they rely on running local-evergreen, which wasn't using the fake Parameter Store implementation (only tests, like make test-model, could use it). To ensure local-evergreen can use the fake implementation rather than rely on a real instance of Parameter Store in AWS, I added a testing-environment-only flag so that local-evergreen can use the fake implementation. Staging/prod won't have the testing environment flag enabled, so they won't use the fake implementation.

Once this is deployed, I'm gonna monitor for errors.

Testing

• Existing tests for GitHub app keys (including Spruce e2e tests for GitHub app settings) still pass.
• Tested in staging that a new project that I did not opt into Parameter Store used Parameter Store for the GitHub app key.

Documentation

N/A