ci: have one builder use ompi 5.0.x devel

Problem: there is no check that flux-pmix continues to
work with the latest ompi 5.0.x development branch.

Make one el8 builder build 5.0.x with the latest required
openpmix.  Don't fail the entire workflow if it fails.

.github/workflows/main.yml

@@ -31,9 +31,10 @@ jobs:
             chain_lint: t
         - name: "el8 - ompi v5.0.x, distcheck"
           image: "el8"
-          ompi_branch: "v5.0.0rc12"
-          openpmix_branch: "v4.2.3"
+          ompi_branch: "v5.0.x"
+          openpmix_branch: "v4.2.4"
           coverage: false
+          continue-on-error: true
           env:
             DISTCHECK: t
         - name: "el8 - pmix v3.2.3, ompi v4.1.4"