.gitlab-ci.yml: Branch-restrict stop_deploy

I'm not sure that's necessary, but I prefer having consistent restrictions for
the two environment-related jobs.
4 jobs for feature-c/ci in 2 minutes and 35 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test Unit
passed #2820
test_browser_unit

00:00:08

 
  Test Functional
passed #2821
test_browser-functional

00:02:16

 
  Deploy
passed #2822
deploy

00:00:06

passed #2823
manual
stop_deploy

00:00:03