CI: Fix visualization random failure

[SMoraisAnsys]

Description

From time to time we end up with visualization tests failing because of some matplotlib calls. This PR adds an environment variable for those tests in order to leverage Agg and avoid failures.

Note, let us see how much time this adds to the CICD though :/

Issue linked

Related to #5524

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.36%. Comparing base (d60364b) to head (af5743f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6346   +/-   ##
=======================================
  Coverage   85.36%   85.36%           
=======================================
  Files         175      175           
  Lines       65798    65798           
=======================================
+ Hits        56170    56171    +1     
+ Misses       9628     9627    -1     

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