[Google][InstanceID] Fixed sample build

build.cake

@@ -50,7 +50,7 @@ Setup (context =>
 {
 	IS_LOCAL_BUILD = string.IsNullOrWhiteSpace (EnvironmentVariable ("AGENT_ID"));
 	Information ($"Is a local build? {IS_LOCAL_BUILD}");
-	BACKSLASH = IS_LOCAL_BUILD ? @"\\" : @"\";
+	BACKSLASH = IS_LOCAL_BUILD ? @"\" : @"\";
 });
 
 Task("build")

samples/Google/InstanceID/InstanceIDSample/InstanceIDSample.csproj

@@ -99,6 +99,7 @@
     </Compile>
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\iOS\Xamarin.iOS.CSharp.targets" />
+  <Import Project="$(MSBuildThisFileDirectory)..\..\..\..\source\Firebase\Core\Core.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\..\..\..\source\Firebase\Analytics\Analytics.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\..\..\..\source\Google\Core\Core.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\..\..\..\source\Google\InstanceID\InstanceID.targets" />