Seems like it better belongs in Cubical.Foundations.Transport, keeping it in a Builtin file makes it seem more mysterious than it is.

...Though maybe it's more mysterious than I think: Is the construction of pathToEquiv in Agda.Builtin.Glue any better than isoToEquiv (pathToIso p) in Cubical.Foundations.Transport? My understanding is that the key property pathToEquiv has is that invEq (pathToEquiv) is definitionally equal to transport⁻ p, but both have this property. Is the fiber contraction of pathToEquiv superior to isoToEquiv (pathToIso p) in some way?