CppWinRT NuGet package deletes a file it doesn't write

[davidmatson]

Snippets from the CppWinRT NuGet package’s targets file:

        <CppWinRTProjectWinMD>$(OutDir)$(RootNamespace).winmd</CppWinRTProjectWinMD>

    <Target Name="CppWinRTClean">
        <ItemGroup>
            <_FilesToDelete Remove="@(_FilesToDelete)"/>
            <_FilesToDelete Include="$(GeneratedFilesDir)**"/>
            <_FilesToDelete Include="$(CppWinRTMergedDir)**"/>
            <_FilesToDelete Include="$(CppWinRTUnmergedDir)**"/>
            <_FilesToDelete Include="$(CppWinRTProjectWinMD)"/> <- The bug is here; this line should be conditional on CppWinRTGenerateWindowsMetadata (otherwise, this targets file is not writing that file and should not delete it)
        </ItemGroup>
        <Delete Files="@(_FilesToDelete)"/>
    </Target>

We noticed this issue when the WinMD was being produced by a different project but just happened to match the value of CppWinRTProjectWinMD.

(The fix is trivial; PR coming shortly.)