chore: remove on: push: branches: main in the workflow

[ycexiao]

What problem does this PR address?

Closes#1

Remove on push to main.

What should the reviewer(s) do?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (974f317) to head (680a3d5).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #29 +/- ##
=======================================
Coverage 94.14% 94.14% =======================================
Files 5 5 Lines 188 188 =======================================
Hits 177 177 Misses 11 11 

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[ycexiao]

@sbillinge, it's ready for review.