feat(rust): add a manifest variable that contains the Cargo.toml values

rust/flake.nix

@@ -27,9 +27,14 @@
 
       systems = ["aarch64-linux" "x86_64-linux" "aarch64-darwin" "x86_64-darwin"];
 
-      perSystem = {pkgs, ...}: let
+      perSystem = {
+        pkgs,
+        lib,
+        ...
+      }: let
         craneLib = crane.mkLib pkgs;
         src = craneLib.cleanCargoSource ./.;
+        manifest = (lib.importTOML "${src}/Cargo.toml").package;
 
         # runtime dependencies
         buildInputs = with pkgs; [
@@ -66,7 +71,7 @@
           programs.alejandra.enable = true;
           programs.rustfmt = {
             enable = true;
-            edition = (builtins.fromTOML (builtins.readFile "${src}/Cargo.toml")).package.edition;
+            edition = manifest.edition;
           };
         };