Re-enable Build step in the future

As of Swift 5.4, building before testing seems to fail:

```
Tests/DynamicCodableTests/DynamicCodableTests.swift:2:18: error: module 'DynamicCodable' was not compiled for testing
@testable import DynamicCodable
~~~~~~~~~~       ^
```

It seems to be down to using the release configuration: `swift build --build-tests --enable-index-store --configuration release`