[ros2]: Fixed preempting of execution

[JafarAbdi]

Same as https://github.com/JafarAbdi/moveit_task_constructor/pull/6

[JafarAbdi]

@mechwiz Please review

[codecov[bot]]

Codecov Report

Merging #363 (fcdce1e) into ros2 (0128cd9) will decrease coverage by 0.04%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             ros2     #363      +/-   ##
==========================================
- Coverage   38.85%   38.82%   -0.03%     
==========================================
  Files          78       78              
  Lines        7520     7526       +6     
==========================================
  Hits         2921     2921              
- Misses       4599     4605       +6     

Impacted Files	Coverage Δ
core/src/task.cpp	61.36% <0.00%> (-1.83%)	:arrow_down:

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[mechwiz]

I tested these changes on a project I'm working on and can verify they work. LGTM!