Prevent a warning: setting Encoding.default_external #3000

Job	Run time
ruby-versions / Generate Ruby versions	3s
vterm-yamatanooroti 2.7 (latest reline)	1m 14s
vterm-yamatanooroti 2.7	1m 17s
rake test 2.7 (latest reline)	56s
vterm-yamatanooroti 3.0 (latest reline)	56s
rake test 2.7	57s
vterm-yamatanooroti 3.0	55s
rake test 3.0 (latest reline)	59s
vterm-yamatanooroti 3.1 (latest reline)	57s
rake test 3.0	54s
vterm-yamatanooroti 3.1	1m 3s
rake test 3.1 (latest reline)	1m 0s
vterm-yamatanooroti 3.2 (latest reline)	55s
rake test 3.1	1m 8s
rake test 3.2 (latest reline)	50s
vterm-yamatanooroti 3.2	1m 9s
rake test 3.2	1m 2s
vterm-yamatanooroti head (latest reline)	47s
rake test head (latest reline)	1m 1s
rake test head	59s
vterm-yamatanooroti head	46s
rake test truffleruby-head (latest reline)	2m 57s
rake test truffleruby-head	2m 59s
	25m 44s

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