Add link to build which trigger data race errors

[rauhul]

SPI is helpfully checking if packages have data race errors and displays that on the package homepage like:

Has data race errors
What is data race safety and how is it tested?

However what wasn't clear is how those builds were run. After speaking with Sven I understood that I needed to look at the swift 6 beta build which enables complete swift-concurrency checking. I think the docs should be updated into include this information somehow.