cloud-init schema should not require sudo

# Bug report

Originally reported by @blackboxsw:

> Confirmed a slightly difference error/warning on cloud-init 24.1~1g1003d438 on Ubuntu Noble. We should sort non-root use of schema validation for files that do not require root read-only permission.
```
ubuntu@nnn:~$ cloud-init schema --config-file ud
2023-12-19 00:50:38,842 - sources[WARNING]: failed loading pickle in /var/lib/cloud/instance/obj.pkl: [Errno 13] Permission denied: '/var/lib/cloud/instance/obj.pkl'
Valid schema ud
ubuntu@nnn:~$ cloud-init --version
/usr/bin/cloud-init 24.1~1g1003d438-0ubuntu1
```

I can confirm that this issue persists on 25.3.