Disable fortification for debug in nix flake

flake.nix

@@ -45,6 +45,12 @@
             ## Dev libs.
             doctest
           ];
+
+          # Nix assumes fortification by default, but that fails with debug builds.
+          # Since this shell is used for development, we disabled fortification. It's
+          # still enabled for our release builds in build.nix.
+          # https://github.com/NixOS/nixpkgs/issues/18995
+          hardeningDisable = [ "fortify" ];
         };
       };
     };