Fix target framework subfolder in the nuget package's build folder

Author: AlexUstinov

…uild/netstandard2.0/Xtensive.Orm.targets …build/netcoreapp3.1/Xtensive.Orm.targetsOrm/Xtensive.Orm/NuGetContent/build/netstandard2.0/Xtensive.Orm.targets renamed to Orm/Xtensive.Orm/NuGetContent/build/netcoreapp3.1/Xtensive.Orm.targets Orm/Xtensive.Orm/NuGetContent/build/netstandard2.0/Xtensive.Orm.targets renamed to Orm/Xtensive.Orm/NuGetContent/build/netcoreapp3.1/Xtensive.Orm.targets

@@ -11,7 +11,7 @@
     <AssemblyTitle>$(Title)</AssemblyTitle>
     <PackageDescription>$(DoProductName) is the object-relational mapper (ORM) and business logic layer (BLL) framework</PackageDescription>
     <SignAssembly>true</SignAssembly>
-    <AssemblyOriginatorKeyFile>..\Orm.snk</AssemblyOriginatorKeyFile>
+    <AssemblyOriginatorKeyFile>$(OrmKeyFile)</AssemblyOriginatorKeyFile>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <WarningLevel>4</WarningLevel>
     <NoWarn>1591</NoWarn>