somehow got `application/vnd.oci.image.layer.v1.tar+gzip` with zstd:chunked annotations

[cgwalters]

I was messing around with zstd:chunked and somehow ended up with this:

skopeo inspect -n docker://quay.io/cgwalters/ostest:zstdchunked
{
    "Name": "quay.io/cgwalters/ostest",
...
    "LayersData": [
        {
            "MIMEType": "application/vnd.oci.image.layer.v1.tar+zstd",
            "Digest": "sha256:b8f741b50c9da370750578605c48ada1a67d01eea3bdae9ce09599c44dad7bfc",
            "Size": 3493473,
            "Annotations": {
                "io.github.containers.zstd-chunked.manifest-checksum": "sha256:01fb915db203d50de642297c259d4be864ee4cf67cd7ec46499d1fba31592820",
                "io.github.containers.zstd-chunked.manifest-position": "2429250:219805:2292554:1",
                "io.github.containers.zstd-chunked.tarsplit-checksum": "sha256:c976711edd7fc3b79ad01e6aceb4e2432f59b93fedbe7abea0e00d08e67b5c2b",
                "io.github.containers.zstd-chunked.tarsplit-position": "2649063:844338:13966956"
            }
        },
        {
            "MIMEType": "application/vnd.oci.image.layer.v1.tar+gzip",
            "Digest": "sha256:5a7e2a9026d1072b92565a640e04337d470fb0d8681563e461b1ecbeb18b548b",
            "Size": 401246016,
            "Annotations": {
                "io.github.containers.zstd-chunked.manifest-checksum": "sha256:3f91e5690651c9a09f9531443396cb3415f96130769192d99688d1102062d865",
                "io.github.containers.zstd-chunked.manifest-position": "400005112:761805:5527253:1",
                "io.github.containers.zstd-chunked.tarsplit-checksum": "sha256:59f55a3be0ef27a9d846773617795ae70279018ec8d2959d57d1456ab2ce3886",
                "io.github.containers.zstd-chunked.tarsplit-position": "400766925:934100:17583014"
            }
        },

There's a mix of gzip and zstd compressed layers; the gzip layers have the zstd-chunked annotations (which is nonsensical obviously).

As far as I can tell, the layers are compressed the way they claim to be (i.e. the gzip layers really are gzip, not zstd; so the bug here seems to be us choosing gzip for some reason (blob caching?))

This is 100% reproducible for me right now, but I haven't dug deep into why it's happening yet.