Add Scala 2.13.15 to compiler plugin cross-build (backport #4410) #6786

Job	Run time
Formatting	3m 12s
ci (ubuntu-20.04, 8, 2.13.10, 2.4, firtool-1.43.0)	12m 47s
ci (ubuntu-20.04, 8, 2.12.17, 2.4, firtool-1.43.0)	13m 1s
compile project with mill	9m 51s
Integration Tests	7m 10s
Test Chisel Scala-CLI Example	3m 18s
Standard Library Tests (2.13.10)	2m 3s
Standard Library Tests (2.12.17)	2m 8s
check tests	1s
all tests passed	1s
publish	0s
	53m 32s

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