fix: move vcpkg.json to subdir (#2821)

This is to get around the following DuckDB CMake warning:
```
CMake Warning at CMakeLists.txt:1340 (message):
  Extension 'vortex' has a vcpkg.json, but build was not run with VCPKG.  If
  build fails, check out VCPKG build instructions in
  'duckdb/extension/README.md' or try manually installing the dependencies in
  .../duckdb-vortexvcpkg.json
```

Author: 0ax1

duckdb-vortex/CMakeLists.txt

@@ -30,7 +30,7 @@ execute_process(
 set(VCPKG_INSTALL_DIR "${CMAKE_BINARY_DIR}/vcpkg_installed")
 file(MAKE_DIRECTORY "${VCPKG_INSTALL_DIR}")
 execute_process(
-    COMMAND "${vcpkg_SOURCE_DIR}/vcpkg" install --no-print-usage --x-install-root=${VCPKG_INSTALL_DIR}
+    COMMAND "${vcpkg_SOURCE_DIR}/vcpkg" install --no-print-usage --x-manifest-root=vcpkg --x-install-root=${VCPKG_INSTALL_DIR}
     WORKING_DIRECTORY "${vcpkg_SOURCE_DIR}"
 )
 set(CMAKE_PREFIX_PATH ${VCPKG_INSTALL_DIR}/arm64-osx)