appimageTools.defaultFhsEnvArgs: move libsecret to targetPkgs (#435048)

pkgs/build-support/appimage/default.nix

···
         krb5
         gsettings-desktop-schemas
         hicolor-icon-theme # dont show a gtk warning about hicolor not being installed
+
+        # libraries not on the upstream include list, but nevertheless expected
+        # by at least one appimage
+        libsecret # For bitwarden, appimage is x86_64 only
       ];
 
     # list of libraries expected in an appimage environment:
···
         at-spi2-core
         pciutils # for FreeCAD
         pipewire # immersed-vr wayland support
-
-        libsecret # For bitwarden
         libmpg123 # Slippi launcher
         brotli # TwitchDropsMiner
       ];