Allow override of DependencyKey value (#8183)

Author: NikolaMilosavljevic

src/Microsoft.DotNet.Build.Tasks.SharedFramework.Sdk/targets/windows/variables.wxi

@@ -23,7 +23,9 @@
     <?error Invalid Platform ($(var.Platform))?>
   <?endif?>
 
-  <?define DependencyKey   = "$(var.DependencyKeyName)_$(var.BuildVersion)_$(var.Platform)$(var.CrossArchContentsPlatformPart)"?>
+  <?ifndef DependencyKey?>
+    <?define DependencyKey = "$(var.DependencyKeyName)_$(var.BuildVersion)_$(var.Platform)$(var.CrossArchContentsPlatformPart)"?>
+  <?endif?>
   <?define DependencyKeyId = "$(var.DependencyKey)" ?>
 
 </Include>