Just a little bit more context.
It appears the version is from feb 18 https://github.com/mattermost/mattermost-server/releases?after=v5.31.3
They recently upgraded from node 12 to node 16 https://github.com/mattermost/mattermost-webapp/commits/master/package-lock.json

The build failure is most likely a dependency problem. (yarn2nix not translating the dependencies just like they are in the package-lock.json). Unfortunately I tried to use node2nix to generate the dependencies. It seems to work fine, up until I try to build it, then I get a segfault (from nix). Maybe it's just on my system.

Here are things that can be tried.

• try node2nix on your system. (you have to run node2nix --development -l package-lock.json to generate the files. Then create a derivation for the dependencies just like in the readme. Very similar to what you are doing with yarn2nix). If it segfaults on your system, then it's really a bug in nix.
• Update the mattermost version first then try again? Like I was saying, there has been heavy changes on the js side since that release. You might get a better chance with the dependencies. I tried on the latest master to install dependencies and build with npm and didn't run in any problems. So if the yarn2nix fails it's a dependency problem.
• Go the Fixed Output Derivation way. We have to remember to actually use node 12. It might not be the prettiest way, but in my experience it will work every time.

You can try disabling the source-map, but if there really is an error in the dependencies generated then you have no idea what you will get after building.

I have a branch here, just adding one commit to show what the build process with node2nix would be like
https://github.com/happysalada/nixpkgs/tree/mattermost

For me running this results in a segfault. If it works on your machine, then you should be able to continue with it.

Just a small update on this.
I've tried with version 5.36.2 of mattermost. Even on local (so completely impure way of doing things) installing with yarn and building I get a failure. I get the same failure in the nix build.
Your approach is the right one here. The problem likely lies on using yarn vs npm.
The problem is that node2nix can't be used since upstream explicitely requires node16 to build and node2nix does not support node16 (I've had problems with it in the past).

It seems to me the only way here would be to go FOD.

Ok, last update, I gave a shot to the FOD with the following expression

  node_modules = stdenv.mkDerivation {
    pname = "${pname}-modules";
    inherit version src;
    buildInputs = [ nodejs-16_x ];
    configurePhase = ''
      export HOME=$(mktemp -d)
    '';
    buildPhase = ''
      npm install
    '';
    installPhase = ''
      cp -r node_modules $out
    '';
    impureEnvVars = lib.fetchers.proxyImpureEnvVars;
    outputHashAlgo = "sha256";
    outputHashMode = "recursive";
    outputHash = "sha256-ID7sStzMln4K+PLplAedC0DOK0q1abqzL6j1u7wDyVI";

    postBuild = ''
      cp -r $out/deps/mattermost-webapp-modules/node_modules/* $out/node_modules/
    '';
  };

the output hash changes, it's not reproducible.

I think this should just be retried on future updates of mattermost, if the yarn vs npm behavior difference persists, perhaps something like npmlock2nix will solve it.

@happysalada do you know what was the source of the hash changes? If you have two different builds and compare them, maybe it turns out that it's something simple to patch?

I don't unfortunately, I never investigated further.