Append -gnu to RUBY_TARGET for GNU Linux builds (#511) #1027

Job	Run time
Bundle install in Dockerfile	1m 6s
oxi-test (ruby=3.0, os=ubuntu-latest)	1m 19s
oxi-test (ruby=3.2, os=ubuntu-latest)	1m 8s
oxi-test (ruby=3.3, os=ubuntu-latest)	1m 18s
oxi-test (ruby=3.0, os=windows-latest)	4m 14s
oxi-test (ruby=3.2, os=windows-latest)	4m 14s
oxi-test (ruby=3.3, os=windows-latest)	3m 37s
oxi-test (ruby=3.0, os=macos-latest)	57s
oxi-test (ruby=3.2, os=macos-latest)	58s
oxi-test (ruby=3.3, os=macos-latest)	1m 14s
magnus-0.5 (ruby=3.0, os=ubuntu-latest)	1m 12s
magnus-0.5 (ruby=3.2, os=ubuntu-latest)	1m 15s
magnus-0.5 (ruby=3.3, os=ubuntu-latest)	1m 26s
magnus-0.5 (ruby=3.0, os=windows-latest)	3m 24s
magnus-0.5 (ruby=3.2, os=windows-latest)	4m 57s
magnus-0.5 (ruby=3.3, os=windows-latest)	5m 53s
magnus-0.5 (ruby=3.0, os=macos-latest)	1m 12s
magnus-0.5 (ruby=3.2, os=macos-latest)	1m 15s
magnus-0.5 (ruby=3.3, os=macos-latest)	1m 14s
magnus-head (ruby=3.0, os=ubuntu-latest)	2m 17s
magnus-head (ruby=3.2, os=ubuntu-latest)	2m 10s
magnus-head (ruby=3.3, os=ubuntu-latest)	2m 9s
magnus-head (ruby=3.0, os=windows-latest)	5m 53s
magnus-head (ruby=3.2, os=windows-latest)	6m 15s
magnus-head (ruby=3.3, os=windows-latest)	6m 23s
magnus-head (ruby=3.0, os=macos-latest)	1m 46s
magnus-head (ruby=3.2, os=macos-latest)	1m 53s
magnus-head (ruby=3.3, os=macos-latest)	1m 46s
	1h 12m 25s

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