use nightly in devshell (#165)

following on from technical discussion, move to using nightly in the
devshell, but keep stable for builds + CI

when running `nix flake update`, it will update to the latest nightly
that has miri

Author: minikin

nix/automation/devshells.nix

@@ -3,11 +3,13 @@
   cell,
 }: let
   inherit (inputs) nixpkgs std;
-  inherit (inputs.cells.lib.toolchains) rustToolchain;
+  inherit (inputs.cells.lib.toolchains) rustNightly;
   l = nixpkgs.lib // builtins;
 
   mkEnv = env: l.mapAttrsToList (name: value: {inherit name value;}) env;
 
+
+
   catalystCore = {...}: {
     name = nixpkgs.lib.mkForce "Catalyst Core";
     env = with nixpkgs;
@@ -25,7 +27,7 @@
     ];
     packages = with nixpkgs; [
       gcc
-      rustToolchain
+      rustNightly
       pkg-config
       protobuf
       uniffi-bindgen

nix/lib/toolchains.nix

@@ -1,17 +1,21 @@
-{
-  inputs,
-  cell,
-}: let
+{ inputs
+, cell
+,
+}:
+let
   inherit (inputs) nixpkgs rust-overlay;
-in rec {
+in
+rec {
   naersk = nixpkgs.callPackage inputs.naersk {
     cargo = rustToolchain;
     rustc = rustToolchain;
   };
   rust-bin =
     (nixpkgs.appendOverlays [
       (import rust-overlay)
-    ])
-    .rust-bin;
+    ]).rust-bin;
   rustToolchain = rust-bin.fromRustupToolchainFile (inputs.self + "/rust-toolchain");
+  rustNightly = rust-bin.selectLatestNightlyWith (toolchain: toolchain.default.override {
+    extensions = [ "rustfmt" "rust-src" "miri" ];
+  });
 }