CI: Fail when find-changed-builds fails

Closes: 2761

.gitlab-ci.yml

@@ -337,7 +337,8 @@ fdroid build:
     - printf "keystore\x3a $keystore\n" >> config.yml
     - chown vagrant $keystore
 
-    - for build in `./tools/find-changed-builds.py`; do
+    - APPS="$(./tools/find-changed-builds.py)"
+    - for build in $APPS; do
           set -x;
           apt-get install sudo;
           chown -R vagrant $home_vagrant;