CI: Use archived RPMs on CentOS 8

CentOS 8 has reached EOL. Packages are no longer mirrored or maintained. A
historic snapshot of the packages is kept on vault.centos.org.

refs #1088, #1090

tests/ansible/hosts/group_vars/all.yml

@@ -1,2 +1,3 @@
 ---
 pkg_mgr_python_interpreter: python
+pkg_repos_overrides: []

tests/ansible/hosts/group_vars/centos8.yml

@@ -1,2 +1,28 @@
 ---
 pkg_mgr_python_interpreter: /usr/libexec/platform-python
+
+pkg_repos_overrides:
+  - dest: /etc/yum.repos.d/CentOS-Linux-AppStream.repo
+    content: |
+      [appstream]
+      name=CentOS Linux $releasever - AppStream
+      baseurl=http://vault.centos.org/$contentdir/$releasever/AppStream/$basearch/os/
+      enabled=1
+      gpgcheck=1
+      gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
+  - dest: /etc/yum.repos.d/CentOS-Linux-BaseOS.repo
+    content: |
+      [baseos]
+      name=CentOS Linux $releasever - BaseOS
+      baseurl=http://vault.centos.org/$contentdir/$releasever/BaseOS/$basearch/os/
+      enabled=1
+      gpgcheck=1
+      gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial
+  - dest: /etc/yum.repos.d/CentOS-Linux-Extras.repo
+    content: |
+      [extras]
+      name=CentOS Linux $releasever - Extras
+      baseurl=http://vault.centos.org/$contentdir/$releasever/extras/$basearch/os/
+      enabled=1
+      gpgcheck=1
+      gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-centosofficial

tests/ansible/hosts/group_vars/debian9.yml

@@ -0,0 +1,4 @@
+pkg_repos_overrides:
+  - dest: /etc/apt/sources.list
+    content: |
+      deb http://archive.debian.org/debian stretch main contrib non-free

tests/ansible/regression/issue_776__load_plugins_called_twice.yml

@@ -10,21 +10,14 @@
     ansible_python_interpreter: "{{ pkg_mgr_python_interpreter }}"
     package: rsync  # Chosen to exist in all tested distros/package managers
   tasks:
-    - name: Switch to centos-stream
-      command: dnf --assumeyes --disablerepo="*" --enablerepo=extras swap centos-linux-repos centos-stream-repos
-      when:
-        - ansible_facts.pkg_mgr in ["dnf"]
-
-    - name: Switch to archive.debian.org
-      # Debian 9 has been archived https://lists.debian.org/debian-devel-announce/2023/03/msg00006.html
+    - name: Switch to archived package repositories
       copy:
-        content: |
-          deb http://archive.debian.org/debian stretch main contrib non-free
-        dest: /etc/apt/sources.list
+        dest: "{{ item.dest }}"
+        content: "{{ item.content }}"
         mode: u=rw,go=r
-      when:
-        - ansible_facts.distribution == "Debian"
-        - ansible_facts.distribution_major_version == "9"
+      loop: "{{ pkg_repos_overrides }}"
+      loop_control:
+        label: "{{ item.dest }}"
 
     - name: Add signing keys
       copy: