Fix IsRidAgnostic fallback logic

[dsplaisted]

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

[Forgind]

I think I was confused last time as far as at what point the $(IsRidAgnostic) is used to set %(IsRidAgnostic). Can you update the comment to be clearer on at what point that happens?

[dsplaisted]

@Forgind

I think I was confused last time as far as at what point the $(IsRidAgnostic) is used to set %(IsRidAgnostic). Can you update the comment to be clearer on at what point that happens?

$(IsRidAgnostic) refers to a property and %(IsRidAgnostic) refers to item metadata. I've updated the comment to try to make it more clear.

[dsplaisted]

@dotnet/msbuild I pushed a new commit that only updated a comment and now a test failed in one leg

[Forgind]

@dsplaisted, we're going to need M2 approval for this, so be ready to solicit it when we have an insertion PR.