nixos/man-db: fix cross‐compilation (#415044)

    environment.systemPackages = [ cfg.package ];

    environment.etc."man_db.conf".text =

      let

        # We unfortunately can’t use the customized `cfg.package` when

        # cross‐compiling. Instead we detect that situation and work

        # around it by using the vanilla one, like the OpenSSH module.

        buildPackage =

          if pkgs.stdenv.buildPlatform.canExecute pkgs.stdenv.hostPlatform then

            cfg.package

          else

            pkgs.buildPackages.man-db;

        manualCache =

          pkgs.runCommand "man-cache"

            {

              nativeBuildInputs = [ cfg.package ];

              nativeBuildInputs = [ buildPackage ];

            }

            ''

              echo "MANDB_MAP ${cfg.manualPages}/share/man $out" > man.conf