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