jobs: Fix builds for GitLab merge requests created from forks

We need to checkout the branch merge-requests/${env.gitlabMergeRequestIid}
in that case.

jobs/templates/swh-pipeline.groovy.j2

+def branch = "master"
+if (env.gitlabMergeRequestIid) {
+  branch = "merge-requests/${env.gitlabMergeRequestIid}"
+} else if (env.gitlabSourceBranch) {
+  branch = env.gitlabSourceBranch
+}
+
 pipeline {
   {% filter indent(width=2) %}
     {%- include 'includes/agent-docker.groovy.j2' -%}
@@ -42,7 +49,7 @@ pipeline {
                       +refs/tags/*:refs/remotes/origin/tags*',
           ]],
           branches: [[
-            name: params.REVISION ?: "${env.gitlabSourceBranch}"
+            name: params.REVISION ?: branch
           ]],
           browser: [
             $class: 'GitLab',

jobs/tools/jenkins-jobs-builder.groovy.j2

+def branch = "master"
+if (env.gitlabMergeRequestIid) {
+  branch = "merge-requests/${env.gitlabMergeRequestIid}"
+} else if (env.gitlabSourceBranch) {
+  branch = env.gitlabSourceBranch
+}
+
 pipeline {
   agent any
 
@@ -8,7 +15,7 @@ pipeline {
         updateGitlabCommitStatus name: 'jenkins', state: 'running'
         checkout([
           $class: 'GitSCM',
-          branches: [[name: env.gitlabSourceBranch ?: "master"]],
+          branches: [[name: branch]],
           userRemoteConfigs: [[
             url: '{{gitlab_url}}/{{gitlab_project_name}}.git',
             refspec: '+refs/heads/*:refs/remotes/origin/* \