feat(@angular/build): rename `experimentalPlatform` to `platform` in … #1779

Job	Run time
list	23s
workflow (build-cli)	31s
workflow (devkit-core-tests-rerun)	27s
workflow (devkit-core-tests)	28s
workflow (modules-builder-tests)	32s
workflow (build-unit-tests)	31s
	2m 52s

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