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>

.github/workflows/main.yml

@@ -227,17 +227,15 @@ jobs:
             pool: ubuntu-latest
           - jobname: linux-sha256
             cc: clang
-            os: ubuntu
             pool: ubuntu-latest
           - jobname: linux-gcc
             cc: gcc
             cc_package: gcc-8
-            pool: ubuntu-latest
+            pool: ubuntu-20.04
           - jobname: linux-TEST-vars
             cc: gcc
-            os: ubuntu
             cc_package: gcc-8
-            pool: ubuntu-latest
+            pool: ubuntu-20.04
           - jobname: osx-clang
             cc: clang
             pool: macos-latest
@@ -282,7 +280,6 @@ jobs:
         - jobname: linux-musl
           image: alpine
         - jobname: linux32
-          os: ubuntu32
           image: daald/ubuntu32:xenial
         - jobname: pedantic
           image: fedora

ci/install-dependencies.sh

@@ -12,7 +12,7 @@ UBUNTU_COMMON_PKGS="make libssl-dev libcurl4-openssl-dev libexpat-dev
  libemail-valid-perl libio-socket-ssl-perl libnet-smtp-ssl-perl"
 
 case "$runs_on_pool" in
-ubuntu-latest)
+ubuntu-*)
 	sudo apt-get -q update
 	sudo apt-get -q -y install language-pack-is libsvn-perl apache2 \
 		$UBUNTU_COMMON_PKGS $CC_PACKAGE

ci/lib.sh

@@ -226,7 +226,7 @@ export GIT_TEST_CLONE_2GB=true
 export SKIP_DASHED_BUILT_INS=YesPlease
 
 case "$runs_on_pool" in
-ubuntu-latest)
+ubuntu-*)
 	if test "$jobname" = "linux-gcc-default"
 	then
 		break