build-aux: Use GitHub repo for LuaJIT
Curl reports that the luajit.org git repo is using a self-signed cert.
This is causing git operations to fail. Let's just use the GitHub
mirror.
Cherry Pick Note:
Manually edited to only change the remote and not the commit.
(cherry picked from commit cfb23a51ff8acad13dc739c31854d9f451e05298)
Ryan Foster