Fix of issue 4051: Nuget.targets broken on mono xbuild

- Don't use property functions on mono xbuild since they are not supported.
- Also, replace <Choose> since again, it's not well supported on mono xbuild.

src/Build/NuGet.targets

@@ -36,21 +36,14 @@
     </PropertyGroup>
 
     <PropertyGroup>
-        <PackagesProjectConfig>$(MSBuildProjectDirectory)\packages.$(MSBuildProjectName.Replace(' ', '_')).config</PackagesProjectConfig>
+        <PackagesProjectConfig Condition=" '$(OS)' == 'Windows_NT'">$(MSBuildProjectDirectory)\packages.$(MSBuildProjectName.Replace(' ', '_')).config</PackagesProjectConfig>
+        <PackagesProjectConfig Condition=" '$(OS)' != 'Windows_NT'">$(MSBuildProjectDirectory)\packages.$(MSBuildProjectName).config</PackagesProjectConfig>
     </PropertyGroup>
 
-    <Choose>
-        <When Condition="Exists('$(PackagesProjectConfig)')">
-            <PropertyGroup>
-                <PackagesConfig>$(PackagesProjectConfig)</PackagesConfig>
-            </PropertyGroup>
-        </When>
-        <When Condition="Exists('$(MSBuildProjectDirectory)\packages.config')">
-            <PropertyGroup>
-                <PackagesConfig>$(MSBuildProjectDirectory)\packages.config</PackagesConfig>
-            </PropertyGroup>
-        </When>
-    </Choose>
+    <PropertyGroup>
+      <PackagesConfig Condition="Exists('$(PackagesProjectConfig)')">$(PackagesProjectConfig)</PackagesConfig>
+      <PackagesConfig Condition="Exists('$(MSBuildProjectDirectory)\packages.config')">$(MSBuildProjectDirectory)\packages.config</PackagesConfig>
+    </PropertyGroup>
 
     <PropertyGroup>
         <!-- NuGet command -->