lotus-bundle: Add arai2c support

[GlacierWalrus]

• Allow user to pass USE_ARIA2C="true" to make the snapshot downloader use aria2c, speediing up the download significantly. Note this will cause a container runtime installation of aria2c before snapshot download. This is less than ideal, but the snapshot downloader would require a lotus-based image including aria2c which is nontrivial to produce.