running rootful buildkitd inside a container in rootless mode: `runc run failed: no cgroup mount found in mountinfo`

### Description

I have a rather complex build system, and for now cannot reduce it to a simple reproducer.

Pretty much, buildkitd is started inside a container, with `--privileged`, and then buildctl is used directly to communicate with it.

When the buildkitd container is started with docker, things work fine.

When the buildkitd container is started with nerdctl (rootless), any RUN instruction fails immediately with:
`runc run failed: no cgroup mount found in mountinfo`

I am not a cgroup specialist... @AkihiroSuda does the above ^ ring a bell for you by any chance? Any specific place I should look into?

If not, I will come up with a reproducer eventually, but need time to simplify it.

### Steps to reproduce the issue

_No response_

### Describe the results you received and expected

na

### What version of nerdctl are you using?

2.0.0

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

None

### Host information

_No response_