Hard coded references to App.config prevents use of config transforms in build.

I'm trying to create a package ExcelDna Xll with a config that is transformed by SlowCheetah before the Dna build/pack steps.

Not really sure how this should be handled. 'Normal' course of action as far as I know is to find app.config by looking at the 'AppConfigWithTargetPath' build property.

Source/ExcelDna.AddIn.Tasks/BuildTaskCommon.cs
```
private string GetAppConfigFileNameAs32Bit()
{
return GetFileNameWithBitnessSuffix("App.config", _fileSuffix32Bit);
}

private string GetAppConfigFileNameAs64Bit()
{
return GetFileNameWithBitnessSuffix("App.config", _fileSuffix64Bit);
}
```