Nix: disable fortify for devshell (#4463)

This disables '_FORTIFY_SOURCE' Werrors trying to compile wlroots.
Long standing issue in NixOS/nixpkgs#60919
afaik.

After this change you should be able to:
```
nix develop
mmeson setup build -Dbuildtype=debug
ninja -C build
```

flake.nix

@@ -86,6 +86,7 @@
           name = "hyprland-shell";
           nativeBuildInputs = with pkgsFor.${system}; [cmake python3];
           buildInputs = [self.packages.${system}.wlroots-hyprland];
+          hardeningDisable = [ "fortify" ];
           inputsFrom = [
             self.packages.${system}.wlroots-hyprland
             self.packages.${system}.hyprland