try to track source-build usage

Author: baronfel

eng/SourceBuildPrebuiltBaseline.xml

@@ -7,4 +7,7 @@
          build because it will be retrieved from the N-1 artifacts. -->
     <UsagePattern IdentityGlob="Microsoft.NET.ILLink.Tasks/8.*" />
   </IgnorePatterns>
-</UsageData>
+  <Usages>
+    <Usage Id="System.Diagnostics.DiagnosticsSource" Version="8.0.0" />
+  </Usages>
+</UsageData>