tests: Fix incorrect prune

Now that we're building a from-scratch image it won't have `/ostree`
in it; this line was always pruning the wrong repo.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/ostree-ext/ci/priv-integration.sh

@@ -112,7 +112,7 @@ systemd-run -dP --wait skopeo copy containers-storage:localhost/fcos-derived "${
 systemd-run -dP --wait skopeo copy "${derived_img}" "${derived_img_dir}"
 
 # Prune to reset state
-ostree refs ostree/container/image --delete
+ostree --repo="${repo}" refs ostree/container/image --delete
 
 repo="${sysroot}/ostree/repo"
 images=$(ostree container image list --repo "${repo}" | wc -l)