Fix several conversions of "ocispec.Image" to "ocispec.Platform"

Several bits of code unmarshal image config JSON into an `ocispec.Image`, and then immediately create an `ocispec.Platform` out of it, but then discard the original image *and* miss several potential platform fields (most notably, `variant`).

Because `ocispec.Platform` is a strict subset of `ocispec.Image`, most of these can be updated to simply unmarshal the image config directly to `ocispec.Platform` instead, which allows these additional fields to be picked up appropriately.

We can use `tianon/raspbian` as a concrete reproducer to demonstrate.

Before:

```console
$ ctr content fetch docker.io/tianon/raspbian:bullseye-slim
...

$ ctr image ls
REF                                     TYPE                                                 DIGEST                                                                  SIZE     PLATFORMS    LABELS
docker.io/tianon/raspbian:bullseye-slim application/vnd.docker.distribution.manifest.v2+json sha256:66e96f8af40691b335acc54e5f69711584ef7f926597b339e7d12ab90cc394ce 28.6 MiB linux/arm/v7 -
```

(Note that the `PLATFORMS` column lists `linux/arm/v7` -- the image itself is actually `linux/arm/v6`, but one of these bits of code leads to only `linux/arm` being extracted from the image config, which `platforms.Normalize` then updates to an explicit `v7`.)

After:

```console
$ ctr image ls
REF                                     TYPE                                                 DIGEST                                                                  SIZE     PLATFORMS    LABELS
docker.io/tianon/raspbian:bullseye-slim application/vnd.docker.distribution.manifest.v2+json sha256:66e96f8af40691b335acc54e5f69711584ef7f926597b339e7d12ab90cc394ce 28.6 MiB linux/arm/v6 -
```

Signed-off-by: Tianon Gravi <admwiggin@gmail.com>
Co-authored-by: Sebastiaan van Stijn <github@gone.nl>

pkg/unpack/unpacker.go

@@ -253,7 +253,7 @@ func (u *Unpacker) unpack(
 	// TODO: Support multiple unpacks rather than just first match
 	var unpack *Platform
 
-	imgPlatform := platforms.Normalize(ocispec.Platform{OS: i.OS, Architecture: i.Architecture})
+	imgPlatform := platforms.Normalize(i.Platform)
 	for _, up := range u.platforms {
 		if up.Platform.Match(imgPlatform) {
 			unpack = up