Support single entry .zips, support .zstd bindists

[amesgen]

See #18 (comment)

This results eg in

    "wasm32-wasi-ghc-gmp-x86_64-darwin": {
        "ghcSubdir": "ghc-9.11.20240519-wasm32-wasi",
        "mirrorUrl": "https://example.org/wasm32-wasi-ghc-gmp-x86_64-darwin.tar.zst",
        "originalUrl": "https://nightly.link/tweag/ghc-wasm-bindists/actions/artifacts/1517260460.zip",
        "sriHash": "sha256-jPlDE353GtKm0xFVknuslplZdCEqMcwLMcHk2GzOr6M="
    },

[TerrorJack]

Nice! Thanks a lot for this; would it be possible to actually run the mirroring logic, without committing change to the json manifest in the main branch? I guess we can actually mirror it once then rewind the change and remove the release, but that sounds a bit too heavyweight.