ci: rework flake integration

Now Use flake part and integrate a build system using nix package building and treefmt-nix as a flake-part module

nix/devshell.nix

@@ -0,0 +1,20 @@
+_: {
+  config.perSystem =
+    {
+      config,
+      inputs',
+      pkgs,
+      ...
+    }:
+    {
+      devShells = {
+        default = pkgs.mkShell {
+          nativeBuildInputs = config.buildPackages ++ config.develPackages ++ config.deployPackages;
+          shellHook = ''
+            ln -snf ${config.pelicanplugins} plugins
+            ln -snf ${config.pelicantheme} theme
+          '';
+        };
+      };
+    };
+}