It would need to be a very good hacker if he managed to install undetectable malware in onlymalware in only 5 minutes. But the danger from a workshop employee isn't the only one, given that your computer was turned-on and even (perhaps) logged-into in an environment that you don't control.
If you are worried,I would suggest to run a few antivirus full scans on your computer, using several well-known such products, in addition to Windows Defender. Some of them can run the scan from the browser.
You may find a list of such products in the article Best Free Antivirus Software.