Add support for experimental flags and add valgrind's --fair-sched

[GuillaumeLagrange]

Fixes #90

$ codspeed run exec -m simulation --skip-upload --experimental-fair-sched -- ls -la

  ⚠ Experimental flags enabled: --experimental-fair-sched
  These may change or be removed without notice.
  Share feedback at https://github.com/CodSpeedHQ/codspeed/issues.


› Running the benchmarks

  ✓  ⚙ CPU Simulation - exec ls -la
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[codspeed-hq]

Merging this PR will not alter performance

✅ 4 untouched benchmarks

---

_{Comparing cod-2418-support-fair-sched-behind-an-unstable-flag (bf8f770) with main (4860c11)}