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)
15 jobs for 565-automatically-trigger-gitlab-pages-pipelines-v9_12 in 10 minutes and 25 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Precheck
passed #70420
amd64 docker linux
precheck:debian:sid:amd64

00:00:18

 
  Build
passed #70421
amd64 docker linux
build:centos:centos6:amd64

00:02:56

passed #70422
amd64 docker linux
build:centos:centos7:amd64

00:03:16

passed #70423
amd64 docker linux
build:debian:sid:amd64

00:02:30

passed #70424
i386 docker linux
build:debian:sid:i386

00:01:38

 
  Test
passed #70425
amd64 docker linux
docs:debian:sid:amd64

00:00:47

passed #70434
amd64 docker linux
install:debian:sid:amd64

00:02:32

passed #70430
amd64 docker linux
systemtest:centos:centos6:amd64

00:06:13

passed #70431
amd64 docker linux
systemtest:centos:centos7:amd64

00:04:44

passed #70432
amd64 docker linux
systemtest:debian:sid:amd64

00:06:18

passed #70433
i386 docker linux
systemtest:debian:sid:i386

00:06:28

passed #70426
amd64 docker linux
unittest:centos:centos6:amd64

00:00:55

passed #70427
amd64 docker linux
unittest:centos:centos7:amd64

00:00:50

passed #70428
amd64 docker linux
unittest:debian:sid:amd64

00:00:47

passed #70429
i386 docker linux
unittest:debian:sid:i386

00:00:54