Include `aria2c` on a-shell mini

`aria2c` is needed for downloading in `ani-cli`, and seems useful to have in general e.g. it’s an optional dependency for `yt-dlp`.

I think it has to be included with a-shell rather than be an optional wasm package, due to:
- It looks harder to build for wasm compared to existing successful iOS builds
- It’s potentially performance sensitive, on iSH downloads are really slow due to the linux emulation (although wasm probably isn’t as bad, will also help with battery life)
- I think pretty useful to have by default on a-shell mini for `ani-cli`/`yt-dlp` use cases

I found an [iOS build script targeting jailbroken devices](https://github.com/MCApollo/aria2/blob/master/compile-ios.sh), I think it should mostly work for a-shell as well.