message: Add a changelog entry that mentions the change in archive file naming.
diff: ### Internals
  -* Lorem ipsum.
  +* The archives produced by the packaging process for Mac builds are now
  + .tar.gz files rather than .tar.xz files, with the exception of the aggregate
  + realm-core-cocoa-VERSION.tar.xz archive, which remains as a .tar.xz file.
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