nixos/test/wpa_supplicant: replace activation script by systemd service

        # Note: secrets are stored outside /etc/ and /nix/store to

        # test for accessibility of these paths

        system.activationScripts.wpa-secrets = {

          deps = [

            "users"

            "specialfs"

        systemd.services.wpa-secrets = {

          wantedBy = [

            "network.target"

            "multi-user.target"

          ];

          text = ''

            install -Dm600 -o wpa_supplicant ${pkgs.writeText "wpa" ''

          before = [

            "network.target"

            "multi-user.target"

          ];

          serviceConfig = {

            Type = "oneshot";

          };

          script =

            let

              secretFile = pkgs.writeText "wpa" ''

                psk_nixos_test=${naughtyPassphrase}

            ''} /var/lib/secrets/wpa

              '';

            in

            ''

              install -Dm600 -o wpa_supplicant ${secretFile} /var/lib/secrets/wpa

            '';

        };