[CI] Archive build reasons (elastic#21347)

Jenkinsfile

@@ -103,18 +103,11 @@ pipeline {
                 mapParallelTasks["${k}"] = v
               }
             }
+            notifyBuildReason()
             parallel(mapParallelTasks)
           }
         }
       }
-      post {
-        always {
-          dir("${BASE_DIR}"){
-            // Archive the markdown files that contain the build reasons
-            archiveArtifacts(allowEmptyArchive: false, artifacts: 'build-reasons/*.md')
-          }
-        }
-      }
     }
   }
   post {
@@ -542,6 +535,17 @@ def isDockerInstalled(){
   }
 }
 
+/**
+* Notify the build reason.
+*/
+def notifyBuildReason() {
+  // Archive the build reason here, since the workspace can be deleted when running the parallel stages.
+  archiveArtifacts(allowEmptyArchive: true, artifacts: 'build-reasons/*.*')
+  if (isPR()) {
+    echo 'TODO: Add a comment with the build reason (this is required to be implemented in the shared library)'
+  }
+}
+
 /**
 * This class is the one used for running the parallel stages, therefore
 * its arguments are passed by the beatsStages step.