Use RuntimeIdentifierGraphPath if available in runtime copy of Microsoft.NET.CrossGen.targets (#91077)

Author: elinor-fung

src/tasks/Crossgen2Tasks/Microsoft.NET.CrossGen.targets

@@ -443,7 +443,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <ResolveReadyToRunCompilers RuntimePacks="@(ResolvedRuntimePack)"
                                 Crossgen2Packs="@(ResolvedCrossgen2Pack)"
                                 TargetingPacks="@(ResolvedTargetingPack)"
-                                RuntimeGraphPath="$(BundledRuntimeIdentifierGraphFile)"
+                                RuntimeGraphPath="$([MSBuild]::ValueOrDefault('$(RuntimeIdentifierGraphPath)', '$(BundledRuntimeIdentifierGraphFile)'))"
                                 NETCoreSdkRuntimeIdentifier="$(NETCoreSdkRuntimeIdentifier)"
                                 EmitSymbols="$(PublishReadyToRunEmitSymbols)"
                                 ReadyToRunUseCrossgen2="$(PublishReadyToRunUseCrossgen2)"