fix: Fix incorrect use of unsigned_abs() for i32 type in rv_trace.rs

[fragchain]

unsigned_abs() was being used on an i32 type, which doesn't exist.
I replaced it with abs() to correctly get the absolute value as i32, and then cast it to u64 as needed.

[moodlezoup]

? https://doc.rust-lang.org/std/primitive.i32.html#method.unsigned_abs