fix(@angular/build): only use external packages for polyfills when no… #2858

Job	Run time
lint	2m 11s
build-e2e-windows	21s
build	23s
e2e-windows	0s
Browser Compatibility Tests	0s
publish-snapshots	0s
test	0s
e2e	0s
e2e-package-managers	0s
e2e-snapshots	0s
	2m 55s

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