Fix Swift 6.2 weak mutability warning in Binder

[kirillyakimovich]

Introduction of weak let additionally added new checks, so the existing pattern warns that weakTarget will never have new value assigned by programmer.

Switching to capture list to get rid of the warning.

Redacted diagnostics message:

..RxSwift/Binder.swift:29:18: weak variable 'weakTarget' was never mutated; consider changing to 'let' constant [#]8;;https://docs.swift.org/compiler/documentation/diagnostics/weak-mutability\WeakMutability]8;;\]

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