This makes the matrix more compact, as we don't need to specify the OS
for the majority of the field (all of them now, but we'll add builds
using "macos-N" later, so it will still be necessary for some of them),
and the builds more stable than using which the current latest Ubuntu
version is.
Note that if we decide to do something for the docs later (e.g. run a
spell checker on them), it should be done in a separate workflow and not
in this one.
