Fix publish error introduced by recent UwpCoreRuntimeSdk work (dotnet/core-setup#2752)

* Modify publish/dir.props to wildcard select the CoreRuntime appx package to upload so it's not uploaded on distros where it has not been built.

* Replace CoreRuntime appx package upload with original statement with an additional Exists check so we don't try to publish the file on distros where we haven't produced it


Commit migrated from https://github.com/dotnet/core-setup/commit/a254d3281536d8cda375fa6f2cbe23ecf90a954b

src/installer/publish/dir.props

@@ -22,7 +22,7 @@
     <RuntimePackageFile Include="$(PackagesOutDir)**/runtime.*.nupkg" >
       <RelativeBlobPath>$(BinariesRelativePath)</RelativeBlobPath>
     </RuntimePackageFile>
-    <UWPArtifactsToUpload Include="$(MicrosoftNetCoreRuntimeAppxOutputPath)" >
+    <UWPArtifactsToUpload Include="$(MicrosoftNetCoreRuntimeAppxOutputPath)" Condition="Exists('$(MicrosoftNetCoreRuntimeAppxOutputPath)')" >
       <RelativeBlobPath>$(BinariesRelativePath)</RelativeBlobPath>
     </UWPArtifactsToUpload>
     <RidAgnosticPackageFile Include="$(PackagesOutDir)**/*.nupkg" Exclude="@(RuntimePackageFile)" >