[XamlC] Disable XamlC on non-Release builds (#1087)

Author: StephaneDelcroix

.nuspec/Microsoft.Maui.Controls.Debug.targets

@@ -1,6 +1,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 	<PropertyGroup>
 		<MauiKeepXamlResources>True</MauiKeepXamlResources>
+		<_MauiForceXamlCForDebug>True</_MauiForceXamlCForDebug>
 	</PropertyGroup>
 
 	<Import Project="$(MSBuildThisFileDirectory)Microsoft.Maui.Controls.targets" />

.nuspec/Microsoft.Maui.Controls.targets

@@ -120,6 +120,7 @@
 		Condition=" '$(DesignTimeBuild)' != 'True' AND '@(MauiXaml)' != ''">
 	    <PropertyGroup>
 		<_MauiXamlCValidateOnly>$(MauiXamlCValidateOnly)</_MauiXamlCValidateOnly>
+		<_MauiXamlCValidateOnly Condition="'$(Configuration)' != 'Release' AND '$(_MauiForceXamlCForDebug)' != 'True'">True</_MauiXamlCValidateOnly>
 		<_MauiXamlCValidateOnly Condition="'$(BuildingForLiveUnitTesting)' == 'True' ">True</_MauiXamlCValidateOnly>
 	    </PropertyGroup>
 		<XamlCTask