fix rpm in the runtime

[matthiasclasen]

Running rpm inside a sandbox with the runtime gives:

$rpm -qa
$

So, something is missing.

[matthiasclasen]

Turns out my problem was that --dbpath requires an absolute path. This works:

ostree --repo=exportrepo checkout -U --subpath=/files/lib/rpm runtime/org.flatpak.Runtime.Var/x86_64/1 var
rpm --dbpath=pwd/var -qa