Fix: set correct 'currently executing' in fork_join task.

This is required to correctly execute multiple, consecutive wait_for_all calls in one fork_join_sub_task.
4 jobs from master in 3 minutes 48 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #2950
build_cmake

00:46

 
  Test
passed #2951
run_tests

00:43

 
  Sanitizer
passed #2953
run_address_sanitizer

01:22

passed #2952
run_thread_sanitizer

00:56