Make it possible to override RUBY_TARGET #1028

Job	Run time
Bundle install in Dockerfile	58s
oxi-test (ruby=3.0, os=ubuntu-latest)	1m 16s
oxi-test (ruby=3.2, os=ubuntu-latest)	1m 33s
oxi-test (ruby=3.3, os=ubuntu-latest)	1m 19s
oxi-test (ruby=3.0, os=windows-latest)	5m 19s
oxi-test (ruby=3.2, os=windows-latest)	5m 21s
oxi-test (ruby=3.3, os=windows-latest)	5m 9s
oxi-test (ruby=3.0, os=macos-latest)	1m 13s
oxi-test (ruby=3.2, os=macos-latest)	1m 21s
oxi-test (ruby=3.3, os=macos-latest)	1m 3s
magnus-0.5 (ruby=3.0, os=ubuntu-latest)	1m 25s
magnus-0.5 (ruby=3.2, os=ubuntu-latest)	1m 27s
magnus-0.5 (ruby=3.3, os=ubuntu-latest)	1m 28s
magnus-0.5 (ruby=3.0, os=windows-latest)	5m 31s
magnus-0.5 (ruby=3.2, os=windows-latest)	5m 38s
magnus-0.5 (ruby=3.3, os=windows-latest)	5m 28s
magnus-0.5 (ruby=3.0, os=macos-latest)	1m 17s
magnus-0.5 (ruby=3.2, os=macos-latest)	1m 15s
magnus-0.5 (ruby=3.3, os=macos-latest)	1m 25s
magnus-head (ruby=3.0, os=ubuntu-latest)	2m 20s
magnus-head (ruby=3.2, os=ubuntu-latest)	2m 19s
magnus-head (ruby=3.3, os=ubuntu-latest)	2m 20s
magnus-head (ruby=3.0, os=windows-latest)	6m 21s
magnus-head (ruby=3.2, os=windows-latest)	6m 35s
magnus-head (ruby=3.3, os=windows-latest)	7m 14s
magnus-head (ruby=3.0, os=macos-latest)	1m 38s
magnus-head (ruby=3.2, os=macos-latest)	1m 55s
magnus-head (ruby=3.3, os=macos-latest)	2m 1s
	1h 22m 9s

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