locally built images are not used if buildkit namespace or nerdctl namespace is not buildkit

### Description

namespace needs to be buildkit to use locally built image as FROM.

### Steps to reproduce the issue

terminal one:

```
sudo /usr/bin/buildkitd --containerd-worker=true --oci-worker=false --containerd-worker-namespace=default
```

terminal two:

```
sudo `which nerdctl` --namespace=default pull alpine:latest
sudo `which nerdctl` --namespace=default build -t alpine:nonexisting1 test1
sudo `which nerdctl` --namespace=default build -t alpine:nonexisting2 test2
```

### Describe the results you received and expected

image should build.

### What version of nerdctl are you using?

```
$ sudo `which nerdctl` version
Client:
Version: v1.6.0
OS/Arch: linux/amd64
Git commit: 5f39d76139344b8aac6fd03cbe0fa2fd50f89929
buildctl:
Version: v0.12.2
GitCommit: 567a99433ca23402d5e9b9f9124005d2e59b8861

Server:
containerd:
Version: 1.6.24
GitCommit: 61f9fd88f79f081d64d6fa3bb1a0dc71ec870523
runc:
Version: 1.1.9
GitCommit: v1.1.9-0-gccaecfc

$ buildctl --version
buildctl github.com/moby/buildkit v0.12.2 567a99433ca23402d5e9b9f9124005d2e59b8861

$ buildkitd --version
buildkitd github.com/moby/buildkit v0.12.2 567a99433ca23402d5e9b9f9124005d2e59b8861
```

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

$ sudo `which nerdctl` info
Client:
Namespace: default
Debug Mode: false

Server:
Server Version: 1.6.24
Storage Driver: overlayfs
Logging Driver: json-file
Cgroup Driver: cgroupfs
Cgroup Version: 1
Plugins:
Log: fluentd journald json-file syslog
Storage: native overlayfs
Security Options:
apparmor
seccomp
Profile: builtin
Kernel Version: 6.1.0-0.deb11.11-amd64
Operating System: Debian GNU/Linux 11 (bullseye)
OSType: linux
Architecture: x86_64
CPUs: 12
Total Memory: 31.28GiB
Name: _______
ID: b0c947b1-ebe4-4934-9ac2-4f8aad5fd613