Generate jar to build gem #42

Job	Run time
ruby-versions / Generate Ruby versions	4s
test (2.5, ubuntu-latest)	48s
test (2.5, macos-latest)	56s
test (2.5, windows-latest)	3m 51s
test (2.6, ubuntu-latest)	35s
test (2.6, macos-latest)	57s
test (2.6, windows-latest)	2m 55s
test (2.7, ubuntu-latest)	41s
test (2.7, macos-latest)	1m 15s
test (2.7, windows-latest)	1m 23s
test (3.0, ubuntu-latest)	19s
test (3.0, macos-latest)	35s
test (3.0, windows-latest)	58s
test (3.1, ubuntu-latest)	20s
test (3.1, macos-latest)	28s
test (3.1, windows-latest)	2m 10s
test (3.2, ubuntu-latest)	25s
test (3.2, macos-latest)	37s
test (3.2, windows-latest)	2m 8s
test (head, ubuntu-latest)	34s
test (head, macos-latest)	38s
test (head, windows-latest)	1m 26s
test (jruby, ubuntu-latest)	4m 17s
test (jruby-head, ubuntu-latest)	4m 27s
test (truffleruby, ubuntu-latest)	2m 52s
test (truffleruby-head, ubuntu-latest)	4m 48s
	40m 27s

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