TrimmerRootDescriptor items are not handled correctly for incremental build

[dsplaisted]

• Create an app with a TrimmerRootDescriptor item pointing to an XML descriptor file with information on what to trim
• Publish the app (with AoT and trimming enabled)
• Modify the XML descriptor file
• Publish again

EXPECTED: ILLink / AoT steps are run again based on the updated descriptor
ACTUAL: Publish output is considered up to date and no changes are made

@dotnet/linker-contrib a new issue has been filed in the ILLink area, please triage

[sbomer]

This appears to be working for PublishTrimmed only, but not PublishAot.