fix shell

day01/.envrc

+use flake

day01/flake.nix

@@ -22,8 +22,7 @@
         defaultPackage = naerskBuildPackage { };
 
         devShell = with pkgs; mkShell {
-          buildInputs = buildInputs ++ [ cargo rustc rustfmt ];
-          RUST_SRC_PATH = rustPlatform.rustLibSrc;
+          buildInputs = [ cargo rustc rustfmt ];
         };
       });
 }