[very low priority] Asterisks in nexmark should be escaped

Nexmark tests echo asterisks. These don't print as asterisks, but instead are expanded by the shell to show the files in the current folder, e.g. "echo src RUN NEXMARK IN BATCH MODE USING DATAFLOW RUNNER src". Which is not really much of an issue but I thought it was kind of silly

https://github.com/apache/beam/blob/f950b71bb37366cf23bab43eb46f77c761e2300b/.test-infra/jenkins/NexmarkBuilder.groovy#L77