Register XStream alias even earlier

[rsandell]

There has been a change in the git plugin that somehow makes GitHubPluginConfig load earlier than GitHubPlugin.start()
So when running the MigratorTest with git plugin 4.12.1 installed (for example during PCT) it fails because the old configuration could not be loaded.

The fix is simply to register the XStream alias before InitMilestone.SYSTEM_CONFIG_LOADED instead of GitHubPlugin.start().

I've verified that it works by temporarily adding the git plugin as a test dependency in the pom of this plugin, and the MigratorTest fails before this change and succeeds after the fix got applied.

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[olamy]

Nice catch!

[Dohbedoh]

Apparently this causes some initialization issue https://issues.jenkins.io/browse/JENKINS-70351.