Update the repo setup for tier0 testing

- Enable debuginfos which is needed for a newly added test [1]
- While here, do not enable epel, it's not needed for tier0

[1] https://gitlab.com/redhat/centos-stream/tests/gcc/-/tree/main/tests/Regression/upstream-libstdcxx-testsuite?ref_type=heads

plans/tier0.fmf

@@ -33,4 +33,4 @@ adjust+:
           - dnf config-manager --set-enabled '*CRB*' || true
           - dnf config-manager --set-enabled crb || true
           - dnf config-manager --set-enabled rhel-buildroot || true
-          - dnf config-manager --set-enabled epel || true
+          - dnf config-manager --set-enabled '*debuginfo*' || true