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CONTRIBUTING.md: Use GitHub Actions build status badge #167

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Job	Run time
tests (2.7, 6.1)	22s
tests (2.7, 7.0)	21s
tests (3.0, 6.1)	13s
tests (3.0, 7.0)	7s
tests (3.1, 6.1)	20s
tests (3.1, 7.0)	11s
tests (3.1, main)	30s
tests (3.2, 6.1)	9s
tests (3.2, 7.0)	10s
tests (3.2, main)	11s
tests (head, 6.1)	14s
tests (head, 7.0)	8s
tests (head, main)	16s
tests (truffleruby, 6.1)	44s
tests (truffleruby, 7.0)	49s
tests (truffleruby, main)	47s
	5m 32s