PVF: fix detection of unshare-and-change-root security capability

One of our security features isn't working on a user's machine. Right now this
feature should be optional, but the bug is that the node detects that the
feature works, so the worker tries to apply it.

The problem is that the detection and application are happening in different
filesystems. Detection tries /tmp whereas application tries the database path.

User reported back:

> there are different permissions for the /tmp/ folder (root ownership) vs the
.local folder (my user account)