[JENKINS-18368]: Decorated Launcher Does Not Maintain "isUnix" for RemoteLauncher

This is a fix to override isUnix on wrapped Launchers when decorating by env or prefix. Currently this causes the state of isUnix on a RemoteLauncher to be lost when environment variables are also specified. 

JIRA: https://issues.jenkins-ci.org/browse/JENKINS-18368

Tests are included to check that the state of isUnix is maintained when wrapping a RemoteLauncher with these two methods.