Run differential ShellCheck on merge_group

[masaru-iritani]

What

Run differential-shellcheck on a merge_group GitHub event, which will be fired when a change is added to the merge queue.

Why

For #433, we want to know what link_to_results would be for the code scanning result triggered by a merge_group event. We need to try running the workflow on a merge_group event (without #433).

How tested

An equivalent change works in our private repository.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.71%. Comparing base (b8c7b83) to head (9b9b0e2).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #434   +/-   ##
=======================================
  Coverage   85.71%   85.71%           
=======================================
  Files           5        5           
  Lines         315      315           
=======================================
  Hits          270      270           
  Misses         45       45           

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[jamacku]

Thank you, LGTM