Fix debug output of plugin class path

[TobiX]

Just a minor nit: Before, there was no separator between elements, with this naive change there is one too much at the end...

[keeganwitt]

Yea, I guess to be consistent with the other classpaths that are logged, probably this would be better (unfortunately this isn't Java 8, for compatibility, so I don't have String.join(), so it's a bit verbose):

for (int i = 0; i < pluginArtifacts.size(); i++) {
    sb.append(pluginArtifacts.get(i).getFile());
    if (i < pluginArtifacts.size() - 1) {
        sb.append(", ");
    }
}

[keeganwitt]

Thanks for the MR!