.gitlab-ci.yml: launch only monitor_app stage here

See #3099.
2 jobs from feature/monitor-app in 2 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Monitor App
failed #257415
web
monitor_app

00:00:02

failed #257414
web
monitor_app

00:00:02

 
Name Stage Failure
failed
monitor_app Monitor App

[Minitest::CI] Generating test report in JUnit XML format...
Makefile:10: recipe for target 'live' failed
make[1]: *** [live] Error 1
make[1]: Leaving directory '/home/gitlab-runner/builds/b8d876c1/0/vidjil/vidjil/server/web2py/applications/vidjil/tests'
Makefile:18: recipe for target 'live' failed
make: *** [live] Error 2
make: Leaving directory '/home/gitlab-runner/builds/b8d876c1/0/vidjil/vidjil/server'
ERROR: Job failed: exit status 1