Change multi-target build

Author: SergeiPavlov

Directory.Build.props

@@ -40,8 +40,9 @@
   <PropertyGroup>
     <NoLogo>true</NoLogo>
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>
-    <TargetFrameworks>net5.0</TargetFrameworks>
-    <TargetFrameworks Condition="'$(DO_TargetFrameworks)' != ''">$(DO_TargetFrameworks)</TargetFrameworks>
+    <TargetFrameworks>$(TargetFrameworks)</TargetFrameworks> <!-- the property -->
+    <TargetFrameworks Condition="'$(TargetFrameworks)'==''">$(DO_TargetFrameworks)</TargetFrameworks> <!-- env var -->
+    <TargetFrameworks Condition="'$(TargetFrameworks)'==''">net5.0</TargetFrameworks> <!-- fallback to default -->
     <LangVersion>9.0</LangVersion>
     <SolutionDir Condition="$(SolutionDir) == ''">$([MSBuild]::EnsureTrailingSlash(
       $([MSBuild]::GetDirectoryNameOfFileAbove('$(MSBuildThisFileDirectory)', 'Orm.sln'))))</SolutionDir>