agda: skip eval errors in allPackages (#502548)

    Takes an arbitrary derivation and says whether it is an agda library package

    *  that is not marked as broken.

  */

  isUnbrokenAgdaPackage = pkg: pkg.isAgdaDerivation or false && !pkg.meta.broken;

  isUnbrokenAgdaPackage =

    pkg:

    let

      r = builtins.tryEval (pkg.isAgdaDerivation or false && !pkg.meta.broken);

    in

    r.success && r.value;

}