Update scala-compiler, scala-library to 2.12.20 #216

Job	Run time
Build docs	1m 6s
Check formatting	53s
Integration tests	1m 28s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.13)	2m 29s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.14)	2m 31s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.15)	2m 36s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.16)	2m 39s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.17)	2m 31s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.18)	2m 31s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.19)	2m 30s
Compiler and SBT plugin tests (ubuntu-latest, 2.12.20)	2m 33s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.5)	2m 28s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.6)	2m 29s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.7)	2m 35s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.8)	2m 39s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.9)	2m 26s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.10)	2m 41s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.11)	2m 27s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.12)	2m 33s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.13)	2m 29s
Compiler and SBT plugin tests (ubuntu-latest, 2.13.14)	2m 39s
	49m 13s

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