github-actions: run gcc-8 on ubuntu-20.04 image

GitHub starts to upgrade its runner image "ubuntu-latest" from version
"ubuntu-20.04" to version "ubuntu-22.04". It will fail to find and
install "gcc-8" package on the new runner image.

Change some of the runner images from "ubuntu-latest" to "ubuntu-20.04"
in order to install "gcc-8" as a dependency.

The first revision of this patch tried to replace "$runs_on_pool" in
"ci/*.sh" with a new "$runs_on_os" environment variable based on the
"os" field in the matrix strategy. But these "os" fields in matrix
strategies are obsolete legacies from commit [1] and commit [2], and
are no longer useful. So remove these unused "os" fields.

[1]: c08bb26010 (CI: rename the "Linux32" job to lower-case "linux32",
                 2021-11-23)
[2]: 25715419bf (CI: don't run "make test" twice in one job, 2021-11-23)

Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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