Make tools/bufr_compare_dir a plain sh script

[ivorblockley]

A very minor simplification - drop the need for ksh from the tools/bufr_compare_dir utility.

At the Bureau of Meteorology we build a custom eccodes internally and our CI that tests this tool fails due to a lack of Korn shell on our test VMs. It's the only non ".ksh" script in the code-base that uses ksh, and it's easy enough to make it work with any POSIX shell, as per this pull request.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.86%. Comparing base (db8d725) to head (de3f598).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #235   +/-   ##
========================================
  Coverage    86.86%   86.86%           
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  Files          784      784           
  Lines        62631    62631           
  Branches     11035    11035           
========================================
  Hits         54406    54406           
  Misses        8225     8225           

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