Re: [PATCH v3 25/29] CI: set CC in MAKEFLAGS directly, don't add it to the environment

> Rather than pass a "$CC" and "$CC_PACKAGE" in the environment to be
> picked up in ci/lib.sh let's instead have ci/lib.sh itself add it
> directly to MAKEFLAGS.
> 
> Setting CC=gcc by default made for confusing trace output, and since a
> preceding change to carry it and others over across "steps" in the
> GitHub CI it's been even more misleading.  E.g. the "win+VS build" job
> confusingly has CC=gcc set, even though it builds with MSVC.
> 
> Let's instead reply on the Makefile default of CC=cc, and only
> override it for those jobs where it's needed. This does mean that
> we'll need to set it for the "pedantic" job, which previously relied
> on the default CC=gcc in case "clang" become the default on that
> platform.

> This partially reverts my 707d2f2fe86 (CI: use "$runs_on_pool", not
> "$jobname" to select packages & config, 2021-11-23), i.e. we're now
> aiming to only set those variables specific jobs need.
> 
> Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>