stdenv: shorten evaluation errors when in Hydra
stdenv: shorten evaluation errors when in Hydra

Hydra's page showing evaluation errors is about a mile long, showing
buckets of user-friendly errors, like this:

    in job ‘seyren.aarch64-linux’:
    Package ‘oraclejre-8u191’ in /nix/store/fa9zzkbljkvdavwzirkrr5irg25ymbjl-source/pkgs/development/compilers/oraclejdk/jdk-linux-base.nix:71 has an unfree license (‘unfree’), refusing to evaluate.

    a) For `nixos-rebuild` you can set
      { nixpkgs.config.allowUnfree = true; }
    in configuration.nix to override this.

    b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
      { allowUnfree = true; }
    to ~/.config/nixpkgs/config.nix.

    in job ‘jetbrains.webstorm.x86_64-linux’:
    Package ‘webstorm-2018.3.1’ in /nix/store/fa9zzkbljkvdavwzirkrr5irg25ymbjl-source/pkgs/applications/editors/jetbrains/default.nix:230 has an unfree license (‘unfree’), refusing to evaluate.

    a) For `nixos-rebuild` you can set
      { nixpkgs.config.allowUnfree = true; }
    in configuration.nix to override this.

    b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
      { allowUnfree = true; }
    to ~/.config/nixpkgs/config.nix.

This makes it extremely hard to find actual issues in the output. This
patch set makes the output much more condensed in Hydra:

    Failed to evaluate nifticlib-2.0.0: «unsupported»: is not supported on ‘x86_64-apple-darwin’
    Failed to evaluate dmd-2.081.2: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
    Failed to evaluate dmdBuild-2.081.2: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
    Failed to evaluate ldc-1.11.0: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
    Failed to evaluate ldcBuild-1.11.0: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
    Failed to evaluate ldc-0.17.5: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’
    Failed to evaluate ldcBuild-0.17.5: «unsupported»: is not supported on ‘aarch64-unknown-linux-gnu’

SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,SymbiFlow/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs