Fix: Always use conda clang-cl

[olivier-roussel]

It appears that GH runners installs a clang-cl version which is the one that is used in our CI when just setting compiler variables

CC=clang-cl
CXX=clang-cl

in our Conda environment even when the clang-cl conda package is installed (see stack-of-tasks/eigenpy#603 ).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 46.97%. Comparing base (08ea849) to head (9fb6480).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1809      +/-   ##
==========================================
- Coverage   51.68%   46.97%   -4.72%     
==========================================
  Files         525      525              
  Lines       75361    68272    -7089     
  Branches    32191    31834     -357     
==========================================
- Hits        38952    32068    -6884     
+ Misses      32271    25775    -6496     
- Partials     4138    10429    +6291     

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