CI: add back EL 8

Add back EL 8 to test legacy compatibility

Follow-up to PR 10297

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

.github/workflows/ci.yml

@@ -528,6 +528,11 @@ jobs:
       matrix:
         box:
           - fedora/39-cloud-base
+          # We have to keep EL8 to test old glibc, cgroup, kernel, etc.
+          # The image was changed from rockylinux/8 to almalinux/8,
+          # as the former one no longer works:
+          # https://github.com/containerd/containerd/pull/10297
+          - almalinux/8
           - rockylinux/9@4.0.0
     env:
       BOX: ${{ matrix.box }}