jobs/templates: Add GitLab suport for debian packages related jobs

SWH migration to GitLab is incoming and we are not using elastic workers yet in production so we still need to build debian packages for swh modules.

So add support for building the debian packages when pushing tags to swh module repositories hosted on GitLab.

---

Migrated from D8994 (view on Phabricator)

jobs/templates/debian/automatic-backport.groovy.j2

@@ -36,7 +36,12 @@ pipeline {
           $class: 'GitSCM',
           branches: [[name: "debian/${params.DESTINATION}"], [name: "refs/tags/${params.GIT_TAG}"]],
           userRemoteConfigs: [[
+            {%- if not gitlab_project %}
             url: "https://forge.softwareheritage.org/source/${repo_name}.git",
+            {%- else %}
+            credentialsId: 'gitlab-public-ci-ssh',
+            url: '{{gitlab_ssh_url}}:{{gitlab_project_name}}.git',
+            {%- endif %}
           ]],
         ])
       }
@@ -126,7 +131,11 @@ pipeline {
         expression { backport_ok }
       }
       steps {
+        {%- if not gitlab_project %}
         sshagent (credentials: ['jenkins-public-ci-ssh']) {
+        {%- else %}
+        sshagent (credentials: ['gitlab-public-ci-ssh']) {
+        {%- endif %}
           script {
             def git_push = 'git push --follow-tags --all'
             if (params.DRY_RUN) {