ci: test container embedding on CentOS stream 8

Now that we have enabled container embedding on RHEL 8, let's
also test it there.
We also pin it for Fedora and RHEL/CS 9 to be able to use the
new `org.osbuild.containers.storage.conf` stage.

.gitlab-ci.yml

@@ -680,9 +680,12 @@ ContainerEmbedding:
   script:
     - schutzbot/deploy.sh
     - /usr/libexec/tests/osbuild-composer/container-embedding.sh
-  variables:
-    RUNNER: aws/centos-stream-9-x86_64
-    INTERNAL_NETWORK: "true"
+  parallel:
+    matrix:
+      - INTERNAL_NETWORK: "true"
+        RUNNER:
+        - aws/centos-stream-8-x86_64
+        - aws/centos-stream-9-x86_64
 
 finish:
   stage: finish