Update Focal vagrant box in CI

Built a new CI image for the staging tests. In this GCP image, the
Ubuntu Focal Vagrant VM has had its version bumped from
202105.25.0 -> 202107.28.0. The latest version, 202109.07.0, would not
install successfully via `vagrant box add`.

devops/gce-nested/gce-start.sh

@@ -25,7 +25,7 @@ function find_latest_ci_image() {
     #    --filter="family:fpf-securedrop AND name ~ ^ci-nested-virt" \
     #    --sort-by=~Name --limit=1 --format="value(Name)"
     # Return hardcoded image id to prevent newer builds from breaking CI
-    echo "ci-nested-virt-buster-1623169910"
+    echo "ci-nested-virt-buster-1633365108"
 }
 
 # Call out to GCE API and start a new instance, designating