If no snapshotter is specified, container run selects the default
snapshotter.
However, if `os.features` is set, we should always call
`checkSnapshotterSupport()`. This ensures containerd clients
report a clear error:
```
ctr: snapshotter overlayfs does not support platform
{amd64 linux [erofs] } for image sha256:[]
```
instead of the confusing layer extraction error:
```
ctr: apply layer error for "": failed to extract layer sha256:[]:
failed to get stream processor for application/vnd.erofs.layer.v1:
no processor for media-type
```
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
940076477e