fix(history): satisfy newer clippy's unnecessary_sort_by lint

CI runs clippy on latest stable (dtolnay/rust-toolchain@stable, no
pin), which is ahead of the local dev toolchain here (1.98.0 vs
1.94.0) and now flags load_sessions's sort_by as better expressed with
sort_by_key — this was pre-existing code, just not caught locally
until CI's toolchain moved past it.

Refs #175