v2.0: Fix TruffleRuby simplecov #304

Job	Run time
rubocop	12s
Unit test (ubuntu-20.04, 3)	18s
Unit test (ubuntu-20.04, 3.1)	17s
Unit test (ubuntu-20.04, 3.2)	23s
Unit test (ubuntu-20.04, 3.3)	17s
Unit test (ubuntu-20.04, jruby-9.4)	1m 2s
Unit test (ubuntu-20.04, truffleruby)	1m 51s
Unit test (macos-latest, 3)	26s
Unit test (macos-latest, 3.1)	29s
Unit test (macos-latest, 3.2)	22s
Unit test (macos-latest, 3.3)	21s
Unit test (macos-latest, jruby-9.4)	37s
Unit test (macos-latest, truffleruby)	1m 44s
Unit test (windows-latest, 3)	1m 48s
Unit test (windows-latest, 3.1)	1m 59s
Unit test (windows-latest, 3.2)	2m 17s
Unit test (windows-latest, 3.3)	4m 51s
finish	2s
	19m 16s

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