ci-fairy: add a command to update labels of a merge request

Often users don't read the CI pipeline results or the junit-parsed test logs
or the job status. The only way I found ppl to pay attention to this is to
attach a label "CI failed - please fix". Let's automate this.

This is intended as an externally-called command, not from within a the
pipeline (because checking a pipeline within a pipeline is a bit pointless).
Think of a cronjob run every hour or so to check the status.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>