Add partial cross-compilation for Scala 3 #3069

Job	Run time
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / compile project with mill	5m 19s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / compile project with mill	5m 19s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / ci	10m 49s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / ci	10m 25s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Benchmarking Sanity Check	2m 45s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Benchmarking Sanity Check	2m 45s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Formatting	1m 32s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Formatting	1m 32s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Integration Tests	12m 28s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Integration Tests	12m 28s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Standard Library (2.13.15)	4m 48s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Standard Library (2.13.15)	4m 48s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Build Mdoc & Website	6m 26s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Build Mdoc & Website	6m 26s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Test Scala-CLI Example / Build and Test	15m 7s
ci (ubuntu-22.04, 8, 2.13.15, 2.4, 7.0) / Test Scala-CLI Example / Build and Test	15m 7s
check tests	1s
check tests	0s
all tests passed	1s
all tests passed	1s
Deploy Website	1s
Deploy Website	0s
publish	1s
publish	0s
	1h 58m 9s

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