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 <[email protected]>
Helped-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Jiang Xin <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jiangxin

.github/workflows/main.yml

@@ -282,7 +282,7 @@ jobs:
             pool: ubuntu-latest
           - jobname: linux-gcc
             cc: gcc
-            pool: ubuntu-latest
+            pool: ubuntu-20.04
           - jobname: osx-clang
             cc: clang
             pool: macos-latest