Fix workflow matrix reference

[matsumotosan]

What does this PR do?

Fixes #<issue_number>

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📚 Documentation preview 📚: https://pytorch-lightning--21145.org.readthedocs.build/en/21145/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (14a57c7) to head (9388198).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #21145    +/-   ##
========================================
- Coverage      87%      87%    -1%     
========================================
  Files         269      269            
  Lines       23568    23568            
========================================
- Hits        20551    20419   -132     
- Misses       3017     3149   +132