[scala3] Enable scala3_sources #674

Job	Run time
sbt scripted tests (gen-scala-server, 2.12)	10m 46s
Compile Benchmarks	1m 17s
Compile and test (2.12.18, 8)	3m 38s
sbt scripted tests (gen-java, 2.12)	3m 31s
Checks	1m 39s
Compile and test (2.12.18, 11)	3m 31s
Compile and test (2.13.12, 8)	4m 11s
Docs	2m 20s
Compile and test (2.13.12, 11)	3m 57s
Gradle tests	3m 39s
Maven tests	2m 51s
Compile and test (3.3.1, 8)	5m 0s
Compile and test (3.3.1, 11)	4m 39s
	50m 59s

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