cmake: bump Avendish to master (per-backend AVND_ENABLE gating + VST3 /MT fix)

Single-backend builds no longer fetch/build other backends' deps (godot-cpp,
the VST3 SDK).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QYPRqP7mhC5GKDJV4WksZ5

Author: jcelerier

CMakeLists.txt

@@ -18,7 +18,7 @@ if(NOT Avendish_FOUND)
   FetchContent_Declare(
     Avendish
     GIT_REPOSITORY "https://github.com/celtera/avendish"
-    GIT_TAG 79da89b4c292933f7915ed813d6d9c2856b50f2a)
+    GIT_TAG 6d08f1ef040da7cb62973859b62b14159c864ffe)
   FetchContent_Populate(Avendish)
   list(APPEND CMAKE_PREFIX_PATH "${avendish_SOURCE_DIR}")
   find_package(Avendish REQUIRED)