Investigate whether image export requests over devlxd should use the instances' project

[markylaing]

When a nested LXD downloads an image, if the image can be found on the host and it is public or cached, it is downloaded over the devlxd socket.

Currently all such requests use the default project, and not the project in which the instance is contained. We should determine if this is always suitable, or whether we should be using the project that the instance is contained within.

See this discussion: https://github.com/canonical/lxd/pull/13730#discussion_r1676007520_