crane: incorrectly uses compressed layer of a cosign .sig file to write OCI image from cache

[AustinAbro321]

Describe the bug

If crane.WriteImage uses an already populated cache when writing a cached cosign OCI image it will try to write image layer compressed. Since the image layer is plaintext and shouldn't be compressed the write image will fail with the error error writing layer: expected blob size 244, but only wrote 205. 205 is the file size received if the layer from the image ghcr.io/stefanprodan/podinfo:sha256-57a654ace69ec02ba8973093b6a786faa15640575fbf0dbb603db55aca2ccec8.sig is run through the internal crane function ReadCloserLevel

To Reproduce

First we write the image and populate the cache, then we delete the image so that on the next write crane will attempt to write again and pull from the cache

func run() error {
    cwd, err := os.Getwd()
    if err != nil {
        return err
    }

    craneDirectory := filepath.Join(cwd, "crane")

    cranePath, err := layout.Write(craneDirectory, empty.Index)
    if err != nil {
        return err
    }
    ref := "ghcr.io/stefanprodan/podinfo:sha256-57a654ace69ec02ba8973093b6a786faa15640575fbf0dbb603db55aca2ccec8.sig"
    img, err := crane.Pull(ref, []crane.Option{}...)
    if err != nil {
        return err
    }
    img = cache.Image(img, cache.NewFilesystemCache(filepath.Join(cwd, "cache")))
    err = cranePath.WriteImage(img)
    if err != nil {
        return err
    }

    _ = os.RemoveAll(craneDirectory)

    err = cranePath.WriteImage(img)
    if err != nil {
        return err
    }
    return nil
}

func main() {
    err := run()
    if err != nil {
        fmt.Println(err)
        return
    }
}

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