Update nixpkgs, add direnv to gitignore (#30292)

This also moves nixpkgs to use `channels.nixos.org` since those tarballs
are 30mb in size as compared to 45mb github ones

Release Notes:

- N/A 

----

cc @P1n3appl3

flake.nix

@@ -2,7 +2,7 @@
   description = "High-performance, multiplayer code editor from the creators of Atom and Tree-sitter";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
+    nixpkgs.url = "https://channels.nixos.org/nixpkgs-unstable/nixexprs.tar.xz";
     rust-overlay = {
       url = "github:oxalica/rust-overlay";
       inputs.nixpkgs.follows = "nixpkgs";