ci: skip cleanup before node allocation

Jenkinsfile

@@ -172,6 +172,15 @@ PY
 
     post {
         success { archiveArtifacts artifacts: 'artifacts/*.apk,artifacts/*.sha256,artifacts/*.json', fingerprint: true }
-        always { cleanWs(deleteDirs: true, notFailBuild: true) }
+        always {
+            script {
+                // A build can be aborted while still waiting for an executor. In that
+                // case Jenkins has no FilePath context and cleanWs would mask the
+                // original result with MissingContextVariableException.
+                if (env.NODE_NAME?.trim()) {
+                    cleanWs(deleteDirs: true, notFailBuild: true)
+                }
+            }
+        }
     }
 }