chore: fix build on latest Rust nightlies

The unstable `once_cell` feature has been partially stabilized. Sadly,
the stabilized API does not include `LazyLock`, which has been gated under
`lazy_cell` instead. `LazyLock` is the struct we are interested in.

For now, let's change our dependency on the `once_cell` feature to `lazy_cell`
instead. Builds will break if `lazy_cell` ever gets stabilized and removed,
so we'll react promptly to that.

Ref: rust-lang/rust#105587

packages/packsquash/src/lib.rs

@@ -9,7 +9,7 @@
 #![feature(if_let_guard)]
 #![feature(iter_intersperse)]
 #![feature(map_entry_replace)]
-#![feature(once_cell)]
+#![feature(lazy_cell)]
 #![feature(nonzero_ops)]
 #![feature(try_find)]
 #![feature(type_alias_impl_trait)]