Nixpkgs is by nature fully reproducible and declarative - that means you can't write a nix package to just build from master branch. You also have to specify the sha256 hash to absolutely make sure the source is the same for everyone.
With flakes, it's a bit easier, because if the package you're trying to build has a flake, you can just update it in your own lockfile and it will be rebuilt from the latest version.
If using something from nixpkgs, you have to wait (or PR yourself) for someone to PR the update, or you can also use overrideAttrs to do a "private" fix.
For example, I'm using the wlsunset program, but wanted to build from a different branch, so I did this:

      (wlsunset.overrideAttrs (old: {
        src = fetchFromSourcehut {
          owner = "~kennylevinsen";
          repo = old.pname;
          rev = "81cfb0b4f8e44db9e5ecb36222a24d53a953e6aa";
          sha256 = "sha256-Lxuhsk4/5EHuKPkBbaRtCCQ/LFvIxyc+VQYEoaVT484=";
        };
      }))

This just changes the source attribute to another commit and another sha hash, and it works fine.