Jenkins: fix msg docs stash and archive

Jenkinsfile

@@ -143,15 +143,15 @@ pipeline {
           }
         }
 
-        stage('msg files') {
+        stage('msg file docs') {
           agent {
             docker { image 'px4io/px4-dev-base-focal:2021-05-04' }
           }
           steps {
-            sh './msg/tools/generate_msg_docs.py -d /tmp/msg_docs'
-            dir('/tmp') {
-              archiveArtifacts(artifacts: '/tmp/msg_docs/*.md')
-              stash includes: '/tmp/msg_docs/*.md', name: 'msg_documentation'
+            sh 'mkdir -p build/msg_docs; ./msg/tools/generate_msg_docs.py -d build/msg_docs'
+            dir('build') {
+              archiveArtifacts(artifacts: 'msg_docs/*.md')
+              stash includes: 'msg_docs/*.md', name: 'msg_documentation'
             }
           }
           post {