The scheduled manual builds are currently producing a manual with broken
search links. The file extension is being set to `undefined` rather than
the expected `.html`.

It turns out that these builds are using `sphinx-build 3.1.2` and
searchtools.js now expects DOCUMENTATION_OPTIONS.LINK_SUFFIX rather than
DOCUMENTATION_OPTIONS.FILE_SUFFIX which is being produced by layout.

Solution: peg sphinx to the required version. Things can break when a later
version becomes available and it has not been tested by us.