(RE-372) Make chattr in ship more targeted

Prior to this commit, we were just chattr'ing everything in the
artifacts directory, kind of willy-nilly. This commit targets the
chattr so that we're only calling it on files we actually shipped,
thereby preventing the potential for a race condition and, ultimately,
for build failures.
Merge pull request #216 from melissaanne/maint/master/re-372

(RE-372) Make chattr in ship more targeted