pin nixos-unstable, update flake.lock

`nixos-unstable` is the rolling release channel of NixOS. The default is
the master branch, which doesn't always have a populated binary cache
and so may result in compiling a bunch of stuff unnecessarily.

flake.nix

@@ -1,6 +1,6 @@
 {
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs";
+    nixpkgs.url = "github:NixOS/nixpkgs?ref=nixos-unstable";
     flake-utils.url = "github:numtide/flake-utils";
 
     fenix = {