[JENKINS-42744] - Update WinSW to 2.0.3 and Windows Agent installer to 1.8

This PR updates WinSW to 2.0.3 in order to fix [JENKINS-42744](https://issues.jenkins-ci.org/browse/JENKINS-42744), which is reported as a critical regression in Jenkins 2.50.

Changelog: https://github.com/kohsuke/winsw/blob/master/CHANGELOG.md#203

Full diff: https://github.com/kohsuke/winsw/compare/winsw-2.0.2...winsw-2.0.3