bpo-36108: Avoid failing the build on race condition in clean (GH-12217)

(cherry picked from commit 2f8f564)

Co-authored-by: Steve Dower <[email protected]>

Author: miss-islington

PCbuild/openssl.props

@@ -23,6 +23,6 @@
     <Copy SourceFiles="@(_SSLDLL)" DestinationFolder="$(OutDir)" />
   </Target>
   <Target Name="_CleanSSLDLL" BeforeTargets="Clean">
-    <Delete Files="@(_SSLDLL->'$(OutDir)%(Filename)%(Extension)')" />
+    <Delete Files="@(_SSLDLL->'$(OutDir)%(Filename)%(Extension)')" TreatErrorsAsWarnings="true" />
   </Target>
 </Project>