libretro: keep the shader caches in the system directory

They followed the user directory, which sits in the save directory when the frontend has one. Shader caches belong to the core and the host's GPU, not to a game's saves: they now always go in <system dir>/dolphin-emu/User/Cache/Shaders/.

Source/Core/DolphinLibretro/Boot.cpp

@@ -305,6 +305,15 @@ bool retro_load_game(const struct retro_game_info* game)
 #endif
 
   UICommon::SetUserDirectory(user_dir);
+  /* Shader caches live in the system directory whatever the save directory is: they belong to
+   * the core and the host's GPU, not to a game's saves. */
+  if (system_dir && *system_dir)
+  {
+    const std::string shader_dir = std::string(system_dir) + DIR_SEP "dolphin-emu" DIR_SEP
+                                   "User" DIR_SEP "Cache" DIR_SEP "Shaders" DIR_SEP;
+    File::CreateFullPath(shader_dir);
+    File::SetUserPath(D_SHADERCACHE_IDX, shader_dir);
+  }
   UICommon::CreateDirectories();
   UICommon::Init();
   Libretro::Log::Init();