bump nightly version requirement of `CONFIG_RUSTC_HAS_UNSAFE_PINNED` to 1.89

[BennoLossin]

#37 added CONFIG_RUSTC_HAS_UNSAFE_PINNED which is enabled on >=1.88.0-nightly, but not all 1.88.0-nightly compilers have the unsafe_pinned feature, thus increase it to 1.89.0-nightly when that becomes available.