Allow clippy::unnecessary_fallible_conversions

This lint is part of Rust 1.75.0, but has a lot of problems with FFI
data types that change size between 32-bit and 64-bit targets. This
makes the lint check far less useful, so we are just globally
suppressing it for now.

Bug: http://b/321303117
Test: m
Change-Id: I39531c961a26f533451d73b025caca02e593a269