try this 
```
(pkgs.nyxt.overrideAttrs (drv: {
      src = drv.src.overrideAttrs (drv: {
        src = pkgs.fetchFromGitHub {
          repo = "nyxt";
          owner = "atlas-engineer";
          rev = "12f5e61307b3d74abe9fdad89235f1afde04c2a2";
          sha256 = "0aj3ghw3xl3inrmmjwm44baxg5mw69lmqva7rcjkkcbdsh5s61xb";
        };
      });
    }))
```