Change nuget package suffix to be "-build-" to mimic format used by dotnet pack

We use dotnet pack with the build number parameter, but nuget pack doesn't
have this. This changes the suffix to mimic the version number so that
the names look similar

appveyor.yml

@@ -64,7 +64,7 @@ after_build:
       if ($Env:APPVEYOR_REPO_TAG -eq "true") {
         nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix=""
       } else {
-        nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix="-build$Env:BUILD"
+        nuget pack src/Discord.Net/Discord.Net.nuspec -OutputDirectory "artifacts" -properties suffix="-build-$Env:BUILD"
       }
     }
 - ps: if ($isWindows) { Get-ChildItem artifacts/*.nupkg | % { Push-AppveyorArtifact $_.FullName -FileName $_.Name } }