Explicitly note whether the package version props is being overridden

It's difficult to tell today whether the package version props is correctly passed and used in some scenarios.  Make it more explicit.

Author: mmitche

build/DownloadPackageVersions.targets

@@ -2,6 +2,8 @@
   <Target Name="DownloadPackageVersionsProps"
     DependsOnTargets="BuildDotnetCliBuildFramework">
 
+    <Message Importance="High" Text="Overriding repository dependency versions with $(PB_PackageVersionPropsUrl)"
+        Condition=" '$(PB_PackageVersionPropsUrl)' != '' " />
     <DownloadFile Uri="$(PB_PackageVersionPropsUrl)$(CoreSetupBlobAccessTokenParam)"
                   DestinationPath="$(OrchestratedPackageVersionsProps)"
                   Condition=" '$(PB_PackageVersionPropsUrl)' != '' " />