This turned out to be a syntax issue. Wrapping the command in '‘s caused ${env.BUILD_NUMBER to be passed instead of its value. I wrapped the whole command in "s and escaped the nested. Works fine now.
sh "curl -v --user user:password --data-binary ${buildDir}package${env.BUILD_NUMBER}.tar -X PUT \"http://artifactory.mydomain.com/artifactory/release-packages/package${env.BUILD_NUMBER}.tar\""