fix(@angular-devkit/build-angular): mark `InjectionToken` as pure for… #1957

Job	Run time
list	24s
workflow (build-cli)	1m 59s
workflow (build-unit-tests)	2m 21s
workflow (modules-builder-tests)	1m 47s
workflow (devkit-core-tests)	1m 16s
workflow (devkit-core-tests-rerun)	1m 25s
	9m 12s

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