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add justfile #2071

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Run time

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Job	Run time
Build	40s
Test (2.3)	56s
Test (2.4)	58s
Test (2.5)	1m 4s
Test (2.6)	56s
Test (2.7)	58s
Test (3.0)	52s
Test (3.1)	54s
Test (3.2)	1m 4s
Test (3.3)	56s
Test (jruby-9.4.0.0)	1m 41s
Test (truffleruby-head)	2m 22s
Publish	0s
	13m 21s