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

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 the runner image of the `linux-gcc` job from "ubuntu-latest" to
"ubuntu-20.04" in order to install "gcc-8" as a dependency.

Reviewed-by: Johannes Schindelin <redacted>
Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Jiang Xin <redacted>
Signed-off-by: Junio C Hamano <redacted>