Fixed CI on AppVeyor

.gitignore

@@ -228,5 +228,6 @@ Resource.Designer.cs
 *.bak
 */nuget_version_override.txt
 /src/crosstargeting_override.props
-/build/tools
-/build/*.nupkg
+/build/tools/**
+!build/tools/packages.config
+/build/*.nupkg

build/tools/packages.config

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<packages>
+    <package id="Cake" version="0.24.0" />
+</packages>