Appveyor build folder isn't constant so use variable in key file path.

Author: ngbrown

appveyor.props.in

@@ -7,6 +7,6 @@
     <PropBuildWinRT>$(BUILDWINRT)</PropBuildWinRT>
     <PropBuildWcf Condition="'$(PropUsingMono)' != 'true'">true</PropBuildWcf>
     <PropAssemblyVersion>$(APPVEYOR_BUILD_VERSION)</PropAssemblyVersion>
-    <PropKeyfile>c:\projects\rabbitmq-dotnet-client\rabbit.snk</PropKeyfile>
+    <PropKeyfile>$(APPVEYOR_BUILD_FOLDER)\rabbit.snk</PropKeyfile>
   </PropertyGroup>
 </Project>