Do we need to set `-YdisableFlatCpCaching` for Scala 3?

[Friendseeker]

PR #826 sets -YdisableFlatCpCaching when outputting Jar on classpath and compiler version is 2.12/2.13.

Do we need to do it for Scala 3? Or do we no longer need it since scala/scala#6064 is merged?

[Friendseeker]

Probably want to go back to the good old approach of... experimentally test original reproduction in #282 and see if the issue is reproducible without -YdisableFlatCpCaching.