Update shell.nix to work with stable nixpkgs

The existing shell.nix uses whatever versions of rustc and cargo are in
the system nixpkgs. In the current stable nixpkgs version (24.11), this
is rustc 1.82.0. Unfortunately, we depend on the `strict_provenance`
feature, which was unstable before 1.84.0. (See: kanidm/concread#132)

This patch makes minimal changes to shell.nix to overlay nixpkgs with
the rustc version defined in rust-toolchain.toml, enabling Kanidm to
build locally on stable versions of NixOS.

shell.nix

@@ -1,14 +1,13 @@
-{ pkgs ? import <nixpkgs> {} }:
-	let
-	overrides = (builtins.fromTOML (builtins.readFile ./rust-toolchain.toml));
+let
+	rust-overlay = (import (builtins.fetchTarball "https://github.com/oxalica/rust-overlay/archive/master.tar.gz"));
 in
-pkgs.mkShellNoCC rec {
+{ pkgs ? import <nixpkgs> { overlays = [ rust-overlay ]; } }:
+pkgs.mkShellNoCC {
 	# Kanidm dependencies
 	buildInputs = with pkgs; [
 		pkg-config
 		
-		cargo
-		rustc
+		(rust-bin.fromRustupToolchainFile ./rust-toolchain.toml)
 
 		clang
 		llvmPackages.bintools
@@ -19,7 +18,6 @@ pkgs.mkShellNoCC rec {
 		linux-pam
 	];
 	
-	RUSTC_VERSION = overrides.toolchain.channel;
 	# https://github.com/rust-lang/rust-bindgen#environment-variables
 	LIBCLANG_PATH = pkgs.lib.makeLibraryPath [ pkgs.llvmPackages_latest.libclang.lib ];
-}
+}