Skip nested IRB tests in Ruby Core CI #2345

Job	Run time
ruby-versions / Generate Ruby versions	2s
rake test 2.7 (latest reline)	50s
rake test 2.7	47s
rake test 3.0 (latest reline)	39s
rake test 3.0	40s
rake test 3.1 (latest reline)	31s
rake test 3.1	32s
rake test 3.2 (latest reline)	32s
rake test 3.2	29s
rake test head (latest reline)	38s
rake test head	28s
rake test truffleruby-head (latest reline)	7m 51s
rake test truffleruby-head	5m 17s
vterm-yamatanooroti 2.7 (latest reline)	1m 33s
vterm-yamatanooroti 2.7	1m 11s
vterm-yamatanooroti 3.0 (latest reline)	42s
vterm-yamatanooroti 3.0	54s
vterm-yamatanooroti 3.1 (latest reline)	55s
vterm-yamatanooroti 3.1	1m 0s
vterm-yamatanooroti 3.2 (latest reline)	51s
vterm-yamatanooroti 3.2	42s
vterm-yamatanooroti head (latest reline)	51s
vterm-yamatanooroti head	45s
	28m 40s

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