Move project repositories to a persistent location

[rjust]

Currently, the init.sh script downloads the project repositories from a URL that is subject to change. Since a URL change will break older releases, the project repositories should be accessible through a persistent link. Once this new URL is established, we should update the URL in older releases.

[Greg4cr]

Closing, as I believe this has been fixed. URL https://defects4j.org/downloads/$ARCHIVE can remain persistent, i.e., by changing the server location behind the scenes.