appveyor: fetch the latest port data

appveyor.yml

@@ -1,3 +1,4 @@
+version: '{branch}.{build}'
 skip_tags: true
 image: Visual Studio 2017
 configuration: Release
@@ -24,6 +25,8 @@ before_build:
         $all_packages += $packages[$i] + ":" + $env:PLATFORM + "-windows-static "
     }
 
+    git -C C:\Tools\vcpkg pull # This is a temporary fix, can be removed after appveyor update its image to include Microsoft/vcpkg#4046
+
     Invoke-Expression -Command "vcpkg install $all_packages"
 build:
   project: build_msvc\bitcoin.sln