ci: pin nightly to 2025-01-02

[littledivy]

Pin the nightly version for now. Starting from nightly 2025-01-03, all cfg(tests) produce compiler error:

error: unexpected `cfg` condition name: `test`
   --> ops/op2/mod.rs:363:7
    |
363 | #[cfg(test)]
    |       ^^^^
    |
    = help: expected names are: `clippy`, `debug_assertions`, `doc`, `docsrs`, `doctest`, `feature`, `fmt_debug`, `miri`, `overflow_checks`, `panic`, `proc_macro`, `relocation_model`, `rustfmt`, `sanitize`, `sanitizer_cfi_generalize_pointers`, `sanitizer_cfi_normalize_integers`, `target_abi`, `target_arch`, `target_endian`, `target_env`, `target_family`, `target_feature`, `target_has_atomic`, `target_has_atomic_equal_alignment`, `target_has_atomic_load_store`, `target_os`, `target_pointer_width`, `target_thread_local`, `target_vendor`, `ub_checks`, `unix`, and `windows`
    = help: consider using a Cargo feature instead
    = help: or consider adding in `Cargo.toml` the `check-cfg` lint config for the lint:
             [lints.rust]
             unexpected_cfgs = { level = "warn", check-cfg = ['cfg(test)'] }
    = help: or consider adding `println!("cargo::rustc-check-cfg=cfg(test)");` to the top of the `build.rs`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg/cargo-specifics.html> for more information about checking conditional configuration
    = note: `-D unexpected-cfgs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(unexpected_cfgs)]`

Seems related to rust-lang/rust#131729 and maybe some changes haven't propagated to cargo yet?