Fix signed file name
Fix signed file name

Fix how we compute the signed file name.

Right now we're computing a property, OutputExtension, based on
OutputType. We're also using another property, OutputName, which doesn't
actually exist as it turns out.

Instead, use TargetFileName.

VitalyTVA/roslyn-analyzers,srivatsn/roslyn-analyzers,qinxgit/roslyn-analyzers,jinujoseph/roslyn-analyzers,SpotLabsNET/roslyn-analyzers,Anniepoh/roslyn-analyzers,mavasani/roslyn-analyzers,pakdev/roslyn-analyzers,natidea/roslyn-analyzers,genlu/roslyn-analyzers,mavasani/roslyn-analyzers,pakdev/roslyn-analyzers,dotnet/roslyn-analyzers,jaredpar/roslyn-analyzers,dotnet/roslyn-analyzers,heejaechang/roslyn-analyzers,bkoelman/roslyn-analyzers,jasonmalinowski/roslyn-analyzers,mattwar/roslyn-analyzers