Automatically trigger GitLab Pages pipelines

Whenever master or one for the v9_* branches gets updated, the current
ARM should be published on GitLab Pages.  Add a pipeline stage which
takes care of triggering GitLab Pages pipelines.  Extend the lifetime of
artifact archives containing the ARM to prevent GitLab Pages pipelines
from failing due to artifacts being unavailable.

(cherry picked from commit 31bde118)
16 jobs for 565-automatically-trigger-gitlab-pages-pipelines-v9_11 in 25 minutes and 14 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed precheck:debian:sid:amd64 #70435
amd64 docker linux

00:00:21

 
  Build
passed build:centos:centos6:amd64 #70436
amd64 docker linux

00:02:29

passed build:centos:centos7:amd64 #70437
amd64 docker linux

00:02:40

passed build:debian:sid:amd64 #70438
amd64 docker linux

00:02:13

passed build:debian:sid:i386 #70439
i386 docker linux

00:01:58

 
  Test
passed docs:debian:sid:amd64 #70440
amd64 docker linux

00:00:52

passed install:debian:sid:amd64 #70449
amd64 docker linux

00:02:31

passed systemtest:centos:centos6:amd64 #70445
amd64 docker linux

00:04:23

passed systemtest:centos:centos7:amd64 #70446
amd64 docker linux

00:05:59

passed systemtest:debian:sid:amd64 #70447
amd64 docker linux

00:10:41

passed systemtest:debian:sid:i386 #70450
linux docker i386

00:10:29

passed unittest:centos:centos6:amd64 #70441
amd64 docker linux

00:01:05

passed unittest:centos:centos7:amd64 #70442
amd64 docker linux

00:01:04

passed unittest:debian:sid:amd64 #70443
amd64 docker linux

00:01:05

passed unittest:debian:sid:i386 #70444
i386 docker linux

00:00:55

failed systemtest:debian:sid:i386 #70448
i386 docker linux

00:11:14