[daemon] Daemon intermittently segfaults on CI unit test run

As can be seen here: https://github.com/canonical/multipass/actions/runs/21045969337/job/60520290393?pr=4614

Core dump points to daemon.cpp line 3790:
```
warning: Unable to find libthread_db matching inferior's thread library, thread debugging will not be available.
Core was generated by `/root/parts/multipass/build/bin/multipass_tests'.
Program terminated with signal SIGSEGV, Segmentation fault.
Downloading source file /root/parts/multipass/src/src/daemon/daemon.cpp
#0 0x0000560bb5fa7f13 in multipass::Daemon::update_manifests_all(bool)::$_34::operator()(std::unique_ptr > const&) const (this=0x7efda40035d0, vm_image_host_ptr=...) at /root/parts/multipass/src/src/daemon/daemon.cpp:3790

```

Start with the CI run and core dump linked in the issue, then inspect src/daemon/daemon.cpp around line 3790 and the update_manifests_all path. Reproduce the multipass_tests failure in CI or locally, identify the intermittent segfault, and confirm that the unit test run completes without it.

Written by the indexing model from the issue text.