masters commit their manifest xml files to the manifest-versions repo, but it's not easy to locate that from the cbuildbot log.  we should upload the final master.xml file like we do for slaves, and we should try to link to the manifest-versions path.

e.g. https://chrome-internal.googlesource.com/chromeos/manifest-versions/+/master/paladin/buildspecs/57/9054.0.0-rc2.xml