ci(elan): update dead repository URLs (#13906)
`Kha/elan` is redirected by github to `leanprover/elan`, but seemingly with a cache that is delayed.
[Zulip](https://leanprover.zulipchat.com/#narrow/stream/113488-general/topic/install.20elan.20fails.20in.20CI/near/280981154)
