x/pkgsite: API docs for tip not showing

[Merovius]

I've had a couple of occasions now, where I was trying to look at package docs for tip, to see the a preview of upcoming API changes. However, lately this has stopped working or is at least very unreliable. For example, when I go to https://pkg.go.dev/testing@master (which is what tip.golang.org/pkg/testing redirects to), I see subdirectories, but no actual API docs:

This seems to be package-dependent, as e.g. https://pkg.go.dev/testing/quick@master works fine.

[seankhliao]

Duplicate of #49229