Prefer let for private configurations

[Coeur]

Prefer let for private configurations.
Otherwise, we may think that the configuration is mutable, which is not really sendable friendly.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[rauhul]

@swift-ci please test

[natecook1000]

Makes sense, thanks @Coeur!