Fix a leak related to use of function used in a closure

[prabhuamol]

There is a leak in fileprivate lazy var loader: DataLoader<CacheKey, Record> because the method implicitly uses self. Making the capture explicit and breaking the retain cycle here. This was also covered in WWDC 2024 analyze heap memory talk https://developer.apple.com/wwdc24/10173?time=1748

Leak detected via instruments

[apollo-cla]

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