Use PATH variable from module system

If taken directly from the environment, it will lose all the additions made by the Nix packages

Accessing the value from the configuration will always improve self-consistency

flake.nix

@@ -22,14 +22,15 @@
       (system: let
         pkgs = nixpkgs.legacyPackages.${system};
         pwd = builtins.getEnv "PWD";
-        path = builtins.getEnv "PATH";
         prefix = "${pwd}/target/prefix";
         lhapath = "${prefix}/share/LHAPDF";
       in {
         default = devenv.lib.mkShell {
           inherit inputs pkgs;
           modules = [
-            ({config, ...}: {
+            ({config, ...}: let
+              path = config.system.path;
+            in {
               packages = with pkgs; [
                 maturin
                 (lhapdf.override {