Output `.json` by default in C3 #6328

Job	Run time
Checks	0s
Add PR to project	3s
Checks	1m 16s
Tests (macos-13, ./fixtures/*)	1m 41s
Tests (macos-13, ./packages/*)	2m 55s
Tests (ubuntu-latest, ./fixtures/*)	46s
Tests (ubuntu-latest, ./packages/*)	50s
Tests (ubuntu-latest, ./tools)	44s
Tests (windows-latest, ./fixtures/*)	1m 37s
Tests (windows-latest, ./packages/*)	1m 54s
	11m 46s

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