gradle - kotlin, multiple exec tasks

Question

Given an Exec gradle task my-custom-cli-task in a gradle kotlin build script defined by

task<Exec>("my-custom-cli-task") {
  workingdir(projectDir)
  commandLine("my-cli-command","somefile.someextension")
}

I need to run this for a set of project files, defined by

val sourcefileSet = fileTree("$projectDir/") { include("**/*.someextension") }

All theses commands could be run in parallel.

How should this be done using gradle kotlin best practices ?

(Note : I'm aware of https://stackoverflow.com/a/43070603/5565079 but It applies to gradle groovy and I'm not sure how it would translate to kotlin.)