ostree-ext: should use ostree api instead of calling the CLI

[jmarrero]

#1917 (review)

When G_MESSAGES_DEBUG is set (e.g., 'all'), GLib and OSTree emit debug messages to stdout instead of stderr. This might corrupt data we parse from ostree's subprocess output.

Switching to the API would have prevented https://issues.redhat.com/browse/OCPBUGS-64692 from happening in the first place.