ci: point C8S containers to the Vault

[tcornell: This has been cherry picked from downstream systemd-rhel8
commit 3aae107]

Temporarily point repos in C8S containers to the Vault (since C8S is
EOL), until we figure out a _proper_ solution.

Related: RHEL-1087

.github/workflows/unit_tests.sh

@@ -137,6 +137,9 @@ for phase in "${PHASES[@]}"; do
 
             # Beautiful workaround for Fedora's version of Docker
             sleep 1
+            # FIXME?: Point C8S repos to the Vault, since C8S is EOL
+            $DOCKER_EXEC bash -xec "sed -i 's/^mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*"
+            $DOCKER_EXEC bash -xec "sed -i 's|#baseurl=http://mirror.centos.org|baseurl=https://vault.centos.org|g' /etc/yum.repos.d/CentOS-*"
             $DOCKER_EXEC dnf makecache
             # Install and enable EPEL
             $DOCKER_EXEC dnf -q -y install epel-release dnf-utils "${ADDITIONAL_DEPS[@]}"