[cmake] explicitly link against stdc++fs if GCC<9

[aaronj0]

Commmit 9d2f4c5 introduced a dependency on std::filesystem which requires explicit linker flags for GCC versions below 9 This fixes a build failure on Alma 8 jobs on ROOT's CI, which still uses GCC8.5

[vgvassilev]

Lgtm!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.40%. Comparing base (af760d0) to head (2783760).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #794   +/-   ##
=======================================
  Coverage   79.40%   79.40%           
=======================================
  Files          11       11           
  Lines        3987     3987           
=======================================
  Hits         3166     3166           
  Misses        821      821           

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