Add Scala 2.13.15 to compiler plugin cross-build (#4410) (#4411) #6788

Job	Run time
Integration Tests (w/ chiseltest)	3m 40s
Standard Library Tests (2.13.10)	1m 53s
Formatting	3m 10s
ci (ubuntu-20.04, 8, 2.13.10, 2.4, firtool-1.37.0)	18m 32s
Standard Library Tests (2.12.17)	1m 53s
ci (ubuntu-20.04, 8, 2.12.17, 2.4, firtool-1.37.0)	18m 24s
Test Chisel Scala-CLI Example	3m 25s
check tests	1s
all tests passed	1s
publish	15m 22s
	1h 6m 21s

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