Auto merge of #18660 - upsuper:var-sub-inherit, r=emilio

Don't use inherited custom properties for substitution

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Custom properties from parent node has been considered in early stage of cascading them (in [`custom_properties::cascade`](https://github.com/servo/servo/blob/01adcf69d794b09396ea469a4fe2ca3324ef9306/components/style/custom_properties.rs#L461-L469)), and there is nothing in the spec stating that substitution should take inherited values into consideration when the value is invalid in the current node. Actually, there is a web-platform test explicitly checks that inherited value is not taken when the value is computed to invalid.

This would fix [bug 1402217](https://bugzilla.mozilla.org/show_bug.cgi?id=1402217).

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