There doesn't seem to be any good reason to use per-language caches, as
there are files that are common to all languages.
But we do want to use different keys for the different compilers and
even different compiler versions, as they yield different results when
compiling the same code.
Set the PATH to include the ccache directory for the subsequent steps,
including the "build" one, rather than only setting it locally for
configure, as this didn't have much effect at all.
Follow the convention of the GitHub built-in steps, such as the first
"Set up job" one and use capitalized verbs in imperative mood rather
than a mix of non-capitalized verbs and nouns.
This is a purely cosmetic change.
Bash is used by default under all platforms we're going to support here,
so there is no need to specify it explicitly. Moreover, by default the
appropriate shell options (such as "pipefail", as well as "e") are used,
and there is no need to override them.
There doesn't seem to be any reason to make them step-specific, so
simplify the workflow by doing it once and only once at the job level.
No real changes.
Don't rely on the auto-generated names which are not very useful: e.g.
currently something like "build (1, python, 3)" is shown, while this
commit changes it to just "python3" which is both shorter and more
readable.
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.
