explicitly export LC_ALL for sphinx builds

[valeriupredoi]

Description

Investigations done in
#2706
our builds suffering from this issue sphinx-doc/sphinx#11739 - no idea why it's reared its head only now, and why it's not manifesting itself in ESMValTool

Closes #2705

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.10%. Comparing base (7a1ffd4) to head (c31ac4c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2708   +/-   ##
=======================================
  Coverage   95.10%   95.10%           
=======================================
  Files         257      257           
  Lines       15021    15021           
=======================================
  Hits        14286    14286           
  Misses        735      735           

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[valeriupredoi]

this is unfortunately not the solution

setting it to LC_ALL=C looks to be doing the part