Fix: GitHub Workflow trigger

[kitsuyui]

GitHub Workflow was triggered only when push event was fired.
Now it is also triggered when pull_request event is fired.
This enables external contributors to run tests when they send pull requests.
(push event is fired when push is done to forked repository, but push event is not fired when push is done to kitsuyui's repository)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (73ebc33) 57.69% compared to head (a008a24) 57.69%.
Report is 11 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #188   +/-   ##
=======================================
  Coverage   57.69%   57.69%           
=======================================
  Files           5        5           
  Lines         104      104           
  Branches       11       11           
=======================================
  Hits           60       60           
  Misses         44       44           

Flag	Coverage Δ
unittests	57.69% <ø> (ø)

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