fix: pack emudeck libretro builds as dual mode

RetroArch_installCores (emuDeckRetroArch.sh) downloads flycast, mame,
melonds, mgba, mupen64plus_next, ppsspp and scummvm as libretro cores
next to their standalone apps. Listed as standalone only, they got one
layout: once Flycast's standalone files moved to flycast/bios/, the
libretro core lost dc/airlbios.zip, dc/naomi2.zip and dc/segasp.zip.
The scraper now writes them as dual_mode_cores.

platforms/emudeck.yml

@@ -66,6 +66,14 @@ standalone_cores:
 - xenia
 - xenianative
 - yuzu
+dual_mode_cores:
+- flycast
+- mame
+- melonds
+- mgba
+- mupen64plus_next
+- ppsspp
+- scummvm
 systems:
   sony-playstation:
     files: