nix flake: upgrade rustc to fix experimental feature compile error

[kasimeka]

i removed useFetchCargoVendor = true; because it's deprecated and its behaviour is enabled by default in recent versions of nixpkgs.

the PR itself fixes the following error by upgrading to today's rust nightly (with nix flake update), instead of an old one from last year.

>readme error[E0658]: use of unstable library feature `path_add_extension`
>    --> crates/lovely-core/src/dump.rs:146:15
>     |
> 146 |     json_path.add_extension("json");
>     |               ^^^^^^^^^^^^^
>     |
>     = note: see issue #127292 <https://github.com/rust-lang/rust/issues/127292> for more information
>     = help: add `#![feature(path_add_extension)]` to the crate attributes to enable