Fixup signing support conditional (#5752)

Author: mmitche

build/common.targets

@@ -4,7 +4,8 @@
   <PropertyGroup>
     <DefineConstants Condition="'$(TargetFramework)' == '$(NETFXTargetFramework)'">$(DefineConstants);IS_DESKTOP</DefineConstants>
     <DefineConstants Condition="'$(TargetFramework)' != '$(NETFXTargetFramework)'">$(DefineConstants);IS_CORECLR</DefineConstants>
-    <DefineConstants Condition="'$(TargetFramework)' != '$(NetStandardVersion)' And '$(TargetFramework)' != '$(NETCoreLegacyTargetFramework)'">$(DefineConstants);IS_SIGNING_SUPPORTED</DefineConstants>
+    <!-- Signing is only supported in net5.0+ and netframework. -->
+    <DefineConstants Condition="'$(TargetFrameworkIdentifier)' == '.NETFramework' Or ('$(TargetFrameworkIdentifier)' == '.NETCoreApp' and $([MSBuild]::VersionGreaterThanOrEquals($(TargetFrameworkVersion), 5.0)))">$(DefineConstants);IS_SIGNING_SUPPORTED</DefineConstants>
   </PropertyGroup>
 
   <!-- Include shared files for netcore projects -->