ci: fix issues with OSMesa installation and env variables set up

[RobPasMue]

Closes #1926

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.39%. Comparing base (2007ebd) to head (477775b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1927   +/-   ##
=======================================
  Coverage   90.39%   90.39%           
=======================================
  Files         114      114           
  Lines        9545     9545           
=======================================
  Hits         8628     8628           
  Misses        917      917           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.