appveyor: use vcpkg installed not whole vcpkg

Try to cache only the `vcpkg/installed` directory instead of the whole
vcpkg directory, which appveyor says is too large to cache.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

appveyor.yml

@@ -26,4 +26,4 @@ build_script:
   - msbuild -m -p:BuildInParallel=true -p:Configuration=Release ALL_BUILD.vcxproj
 
 cache:
-  - c:\projects\vcpkg
+  - c:\projects\vcpkg\installed