ci: build ompi 2.x, 3.x in el8

Problem: builds of ompi 2.x and 3.x are failing in jammy,
with configure bomging out with a syntax error:

checking for struct sockaddr_in... yes
checking if --with-cuda is set... not set (--with-cuda=)
./configure: line 13031: syntax error near unexpected token `)'
./configure: line 13031: `    )'

Move those builds to el8.

.github/workflows/main.yml

@@ -67,14 +67,14 @@ jobs:
           openpmix_branch: "v4.2.2"
           coverage: false
           env: {}
-        - name: "jammy - ompi v3.1.x"
-          image: "jammy"
+        - name: "el8 - pmix v4.2.2, ompi v3.1.x"
+          image: "el8"
           ompi_branch: "v3.1.x"
           openpmix_branch: "v4.2.2"
           coverage: false
           env: {}
-        - name: "jammy - ompi v3.0.x"
-          image: "jammy"
+        - name: "el8 - pmix v4.2.2, ompi v3.0.x"
+          image: "el8"
           ompi_branch: "v3.0.x"
           openpmix_branch: "v4.2.2"
           coverage: false