.redhat-ci.yml: use new build key

This allows us to more concisely separate building from testing, which
in turn gives us a nicer inheritance pattern in our case.

See also: https://github.com/jlebon/redhat-ci/issues/11

Closes: #569
Approved by: cgwalters

.redhat-ci.Dockerfile

@@ -9,6 +9,8 @@ RUN dnf install -y \
         fuse \
         gjs \
         parallel \
+        clang \
+        libubsan \
         gnome-desktop-testing \
         redhat-rpm-config \
         elfutils \