ci: Separate image preparation commands

The Debian image preparation is moved to before_script to separate it
from the build and test commands. Note that this adds an implicit 5 minute
time limit on this part of the script (see
https://gitlab.com/gitlab-org/gitlab-runner/-/issues/2716).

The commands are also now written once and copied using YAML anchors
(as documented at
https://docs.gitlab.com/ee/ci/yaml/README.html#yaml-anchors-for-before_script-and-after_script).

.gitlab-ci.yml

+.debian_image_preparation: &debian_image_preparation
+  - echo "" | sudo -S apt-get -qq purge -y pd-l2ork > /dev/null || echo "couldn't purge..."
+  - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y update > /dev/null
+  - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y upgrade > /dev/null
+
 debian_stretch_x86_64_deb:
    tags:
      - debian
      - stretch
      - x86_64
+   before_script:
+     - *debian_image_preparation
    script:
-     - echo "" | sudo -S apt-get -qq purge -y pd-l2ork > /dev/null || echo "couldn't purge..."
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y update > /dev/null
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y upgrade > /dev/null
      - cd l2ork_addons
      - ./tar_em_up.sh -B
      - cd ..
@@ -24,10 +28,9 @@ ubuntu_16.04_x86_64_deb:
      - ubuntu
      - v16.04
      - x86_64
+   before_script:
+     - *debian_image_preparation
    script:
-     - echo "" | sudo -S apt-get -qq purge -y pd-l2ork > /dev/null || echo "couldn't purge..."
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y update > /dev/null
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y upgrade > /dev/null
      - cd l2ork_addons
      - ./tar_em_up.sh -B
      - cd ..
@@ -99,10 +102,9 @@ raspbian_stretch_armv7l_deb:
      - raspbian
      - arm 
      - stretch
+   before_script:
+     - *debian_image_preparation
    script:
-     - echo "" | sudo -S apt-get -qq purge -y pd-l2ork > /dev/null || echo "couldn't purge..."
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y update > /dev/null
-     - echo "" | sudo -S DEBIAN_FRONTEND=noninteractive apt-get -qq -y upgrade > /dev/null
      - echo `pwd`
      - cd l2ork_addons
      - ./tar_em_up.sh -r