elan: put lean release in leanprover/lean4

[alissa-tung]

cc @PhotonQuantum

after this PR, please release a new version, and I will bump the docker unified one and change the retain versions to a smaller number.

% tree .
.
├── elan
│   ├── mirror_clone_list.html
│   └── releases
│       ├── download
│       │   ├── mirror_clone_list.html
│       │   └── v3.0.0
│       │       ├── elan-aarch64-apple-darwin.tar.gz
│       │       ├── elan-aarch64-unknown-linux-gnu.tar.gz
│       │       ├── elan-x86_64-apple-darwin.tar.gz
│       │       ├── elan-x86_64-pc-windows-msvc.zip
│       │       ├── elan-x86_64-unknown-linux-gnu.tar.gz
│       │       └── mirror_clone_list.html
│       └── mirror_clone_list.html
├── leanprover
│   └── lean4
│       └── releases
│           └── download
│               └── v4.1.0
│                   ├── lean-4.1.0-darwin.tar.zst
│                   ├── lean-4.1.0-darwin.zip
│                   ├── lean-4.1.0-darwin_aarch64.tar.zst
│                   ├── lean-4.1.0-darwin_aarch64.zip
│                   ├── lean-4.1.0-linux.tar.zst
│                   ├── lean-4.1.0-linux.zip
│                   ├── lean-4.1.0-linux_aarch64.tar.zst
│                   └── lean-4.1.0-linux_aarch64.zip
└── mirror_clone_list.html