Docs Build in CI failing with "failed to load external entity"

Hi,

I know in the past docs builds failing with "failed to load external
entity" have happened on macos. But, recently I've noticed this
failure for docs build on CI (not on macos) -- docs build is one of
the jobs run under the "Compiler Warnings" task.

See an example of this on CI for the github mirror [1].

Anyone know what the story is here? I couldn't find an existing thread
on this specific issue.

- Melanie

[1] https://github.com/postgres/postgres/runs/32028560196