Fix IsRidAgnostic fallback logic (#8002)

Fixes #7995

Context
#6924 changed how IsRidAgnostic works. Normally, the .NET SDK should set the IsRidAgnostic property (added in dotnet/sdk#21986). But when using an older version of the .NET SDK, there is fallback logic in the GetTargetFrameworksWithPlatformForSingleTargetFramework target to replicate the previous logic.

However, this fallback logic was incorrect, as it was setting item metadata, but reading from the property to determine whether to set it to the default value. So if the property wasn't set, the IsRidAgnostic metadata would always be false.

Testing
Manual testing

Author: dsplaisted

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -1964,14 +1964,16 @@ Copyright (C) Microsoft Corporation. All rights reserved.
         <TargetPlatformMonikers Condition="'$(TargetPlatformMoniker)' == ''">None</TargetPlatformMonikers>
         <AdditionalPropertiesFromProject>$(_AdditionalTargetFrameworkInfoProperties)</AdditionalPropertiesFromProject>
 
-        <!-- Determine whether a project is "RID agnostic" for each TargetFramework.  "RID agnostic" means that global properties such as SelfContained and RuntimeIdentifier should
-             not flow across project references.
-
-             Generally this value will come from the IsRidAgnostic property set by the .NET SDK.  If that's not set, then the fallback logic here will be that the project
-             is RID agnostic if it doesn't have RuntimeIdentifier or RuntimeIdentifiers properties set. -->
+        <!-- Determine whether a project is "RID agnostic" for each TargetFramework.  "RID agnostic" means that global properties such as
+             SelfContained and RuntimeIdentifier should not flow across project references.  The IsRidAgnostic metadata value is consumed in the
+             _GetProjectReferenceTargetFrameworkProperties target, where those properties are added to a project's UndefineProperties if
+             IsRidAgnostic is set.
+        
+             Generally we set the IsRidAgnostic metadata based on the IsRidAgnostic property set by the .NET SDK.  If that's not set, then the
+             fallback logic here will be that the project is RID agnostic if it doesn't have RuntimeIdentifier or RuntimeIdentifiers properties set. -->
         <IsRidAgnostic>$(IsRidAgnostic)</IsRidAgnostic>
-        <IsRidAgnostic Condition=" '$(IsRidAgnostic)' == '' and '$(RuntimeIdentifier)' == '' and '$(RuntimeIdentifiers)' == '' ">true</IsRidAgnostic>
-        <IsRidAgnostic Condition=" '$(IsRidAgnostic)' == ''">false</IsRidAgnostic>
+        <IsRidAgnostic Condition=" '%(IsRidAgnostic)' == '' and '$(RuntimeIdentifier)' == '' and '$(RuntimeIdentifiers)' == '' ">true</IsRidAgnostic>
+        <IsRidAgnostic Condition=" '%(IsRidAgnostic)' == ''">false</IsRidAgnostic>
 
       </_TargetFrameworkInfo>
     </ItemGroup>