CI: select CC based on CC_PACKAGE (again)

Fix a regression in 707d2f2fe8 (CI: use "$runs_on_pool", not
"$jobname" to select packages & config, 2021-11-23).

In that commit I changed CC=gcc from CC=gcc-9, but on OSX the "gcc" in
$PATH points to clang, we need to use gcc-9 instead. Likewise for the
linux-gcc job CC=gcc-8 was changed to the implicit CC=gcc, which would
select GCC 9.4.0 instead of GCC 8.4.0.

Furthermore in 25715419bf (CI: don't run "make test" twice in one
job, 2021-11-23) when the "linux-TEST-vars" job was split off from
"linux-gcc" the "cc_package: gcc-8" line was copied along with
it, so its "cc_package" line wasn't working as intended either.

As a table, this is what's changed by this commit, i.e. it only
affects the linux-gcc, linux-TEST-vars and osx-gcc jobs:

	|-------------------+-----------+-------------------+-------+-------|
	| jobname           | vector.cc | vector.cc_package | old   | new   |
	|-------------------+-----------+-------------------+-------+-------|
	| linux-clang       | clang     | -                 | clang | clang |
	| linux-sha256      | clang     | -                 | clang | clang |
	| linux-gcc         | gcc       | gcc-8             | gcc   | gcc-8 |
	| osx-clang         | clang     | -                 | clang | clang |
	| osx-gcc           | gcc       | gcc-9             | clang | gcc-9 |
	| linux-gcc-default | gcc       | -                 | gcc   | gcc   |
	| linux-TEST-vars   | gcc       | gcc-8             | gcc   | gcc-8 |
	|-------------------+-----------+-------------------+-------+-------|

Reported-by: Carlo Arenas <carenas@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ci/lib.sh

@@ -122,7 +122,7 @@ then
 	test macos != "$CI_OS_NAME" || CI_OS_NAME=osx
 	CI_REPO_SLUG="$GITHUB_REPOSITORY"
 	CI_JOB_ID="$GITHUB_RUN_ID"
-	CC="${CC:-gcc}"
+	CC="${CC_PACKAGE:-${CC:-gcc}}"
 	DONT_SKIP_TAGS=t
 
 	cache_dir="$HOME/none"