AppVeyor on_success script shouldn't run in both Windows and Linux jobs

[jonorossi]

The on_success script is used to publish the NuGet package to nuget.org, we only need it to run on Windows, it also fails on Linux anyway:

...
Running "on_success" scripts
if ($env:APPVEYOR_REPO_TAG -eq "true") {
  nuget push ".\build\NVelocity.${env:APPVEYOR_BUILD_VERSION}.nupkg" -ApiKey $env:NUGET_API_KEY -Source https://api.nuget.org/v3/index.json
}
File does not exist (.\build\NVelocity.1.2.0.nupkg).
Command exited with code 1
Build failed