Add constant to make clear usage #87

Job	Run time
test (ubuntu-latest, 2.7)	29s
test (ubuntu-latest, 3.0)	13s
test (ubuntu-latest, 3.1)	16s
test (ubuntu-latest, head)	14s
test (ubuntu-latest, jruby)	56s
test (ubuntu-latest, jruby-head)	1m 4s
test (ubuntu-latest, truffleruby)	43s
test (ubuntu-latest, truffleruby-head)	1m 52s
test (macos-latest, 2.7)	1m 14s
test (macos-latest, 3.0)	42s
test (macos-latest, 3.1)	22s
test (macos-latest, head)	23s
test (macos-latest, jruby)	1m 50s
test (macos-latest, jruby-head)	2m 19s
test (macos-latest, truffleruby)	1m 36s
test (macos-latest, truffleruby-head)	2m 43s
	16m 56s

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