fix coverage artifact handling #41

Job	Run time
test (ubuntu-latest, 2.5)	22s
test (ubuntu-latest, 2.6)	27s
test (ubuntu-latest, 2.7)	30s
test (ubuntu-latest, 3.0)	11s
test (ubuntu-latest, 3.1)	7s
test (ubuntu-latest, 3.2)	9s
test (ubuntu-latest, head)	10s
test (ubuntu-latest, debug)	29s
test (ubuntu-latest, truffleruby)	36s
test (ubuntu-latest, truffleruby-head)	46s
test (ubuntu-latest, jruby)	44s
test (ubuntu-latest, jruby-head)	44s
test (macos-latest, 2.5)	51s
test (macos-latest, 2.6)	1m 5s
test (macos-latest, 2.7)	2m 17s
test (macos-latest, 3.0)	1m 6s
test (macos-latest, 3.1)	21s
test (macos-latest, 3.2)	20s
test (macos-latest, head)	22s
test (macos-latest, debug)	44s
test (macos-latest, truffleruby)	2m 31s
test (macos-latest, truffleruby-head)	1m 48s
test (macos-latest, jruby)	1m 3s
test (macos-latest, jruby-head)	1m 9s
coveralls	4s
	18m 56s

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