Update release workflow to store XCFramework as Zip file

[mattt]

#17299 updated the release workflow to produce .tar.gz archives instead of .zip files. However, Swift Package Manager requires XCFramework binary artifacts to be Zip files:

To distribute code in binary form as a Swift package, create an XCFramework bundle, or artifact, that contains the binaries. Then, make the bundle available locally or on a server:

• When you host the binaries on a server, create a ZIP archive with the XCFramework in its root directory and make it available publicly.

Currently, if you try to include llama.cpp in a Swift project...

    targets: [
        .binaryTarget(
            name: "llama-cpp",
            url:
                "https://github.com/ggml-org/llama.cpp/releases/download/b7503/llama-b7503-xcframework.tar.gz",
            checksum: "7d65a78175b71132401efc99b3898cf6b3779e9782c5d3cbc6e123108b91807b"
        ),
   ]

...you get the following error:

unsupported extension for binary target 'llama-cpp'; valid extensions are: 'artifactbundleindex', 'zip'

This PR partially reverts 7b6d745, updating the release workflow to package XCFramework artifacts as Zip files once again, and adds a comment documenting this requirement.

[CISC]

Thanks for noticing!

[mattt]

Thanks for your quick review, @CISC.

By the way — if any ggml-org owners are interested, I'd be very happy to donate llama.swift, which I created to solve a specific impedance mismatch between Swift Package Manager's implementation of semantic versioning and llama.cpp releases. Just let me know ✌️

[taronaeo]

Oops, thank you!