Fix TOCTOU in create_dirs_impl

[nextsilicon-itay-bookstein]

Replace the check whether a directory exists before trying to
create it with handling for already-exists cases. This addresses
a race where the directory is created by another process, leading
to a failure.

This happened when multiple processes were started up simultaneously with configurations that lead to the creation of the same parent directory.

[codecov]

Codecov Report

Merging #73 (0162cc0) into master (a991861) will decrease coverage by 0.35%.
The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master      #73      +/-   ##
==========================================
- Coverage   88.16%   87.80%   -0.36%     
==========================================
  Files           4        4              
  Lines         566      566              
==========================================
- Hits          499      497       -2     
- Misses         67       69       +2     

Impacted Files	Coverage Δ
include/spdlog_setup/details/conf_impl.h	89.63% <62.50%> (-0.52%)	⬇️

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