[core] cleanup workaround using NIOLockedValueBox instead of Mutex

Due to 
https://github.com/swiftlang/swift/issues/80036
https://github.com/swiftlang/swift/issues/78048

we use `NIOLockedValueBox` instead of `Mutex` in `LambdaRuntime.swift`

This issue to is keep track of that and to not forget remove `NIOLockedValueBox` when 6.0.x will fix this or when we will not support 6.0 anymore (at the release of 6.3 in 2026) 