While there have been malicious packages found in the AUR before[0], I haven't heard of a cryptominer sneaking in... yet.

One was found in the Ubuntu Snap store at one point though[1]

[0]:https://www.bleepingcomputer.com/news/security/malware-found...

[1]:https://www.bleepingcomputer.com/news/linux/malicious-packag...