Append -gnu to RUBY_TARGET for GNU Linux builds #1026

Job	Run time
Bundle install in Dockerfile	1m 7s
oxi-test (ruby=3.0, os=ubuntu-latest)	1m 20s
oxi-test (ruby=3.2, os=ubuntu-latest)	1m 34s
oxi-test (ruby=3.3, os=ubuntu-latest)	1m 6s
oxi-test (ruby=3.0, os=windows-latest)	4m 21s
oxi-test (ruby=3.2, os=windows-latest)	4m 20s
oxi-test (ruby=3.3, os=windows-latest)	5m 9s
oxi-test (ruby=3.0, os=macos-latest)	55s
oxi-test (ruby=3.2, os=macos-latest)	54s
oxi-test (ruby=3.3, os=macos-latest)	1m 4s
magnus-0.5 (ruby=3.0, os=ubuntu-latest)	1m 13s
magnus-0.5 (ruby=3.2, os=ubuntu-latest)	1m 29s
magnus-0.5 (ruby=3.3, os=ubuntu-latest)	1m 30s
magnus-0.5 (ruby=3.0, os=windows-latest)	3m 59s
magnus-0.5 (ruby=3.2, os=windows-latest)	4m 5s
magnus-0.5 (ruby=3.3, os=windows-latest)	3m 46s
magnus-0.5 (ruby=3.0, os=macos-latest)	57s
magnus-0.5 (ruby=3.2, os=macos-latest)	1m 23s
magnus-0.5 (ruby=3.3, os=macos-latest)	1m 2s
magnus-head (ruby=3.0, os=ubuntu-latest)	2m 25s
magnus-head (ruby=3.2, os=ubuntu-latest)	2m 29s
magnus-head (ruby=3.3, os=ubuntu-latest)	2m 14s
magnus-head (ruby=3.0, os=windows-latest)	6m 13s
magnus-head (ruby=3.2, os=windows-latest)	5m 45s
magnus-head (ruby=3.3, os=windows-latest)	6m 13s
magnus-head (ruby=3.0, os=macos-latest)	1m 53s
magnus-head (ruby=3.2, os=macos-latest)	1m 33s
magnus-head (ruby=3.3, os=macos-latest)	1m 34s
	1h 11m 33s

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