Use ex rebuild to pull from COPR

This works around coreos/rpm-ostree#4037

Dockerfile

@@ -6,6 +6,7 @@ ARG FEDORA_COREOS_VERSION=412.36.0
 WORKDIR /go/src/github.com/openshift/okd-machine-os
 COPY . .
 COPY --from=artifacts /srv/repo/*.rpm /tmp/rpms/
+ADD overrides.yaml /etc/rpm-ostree/origin.d/overrides.yaml
 RUN cat /etc/os-release \
     && rpm-ostree --version \
     && ostree --version \
@@ -22,10 +23,7 @@ RUN cat /etc/os-release \
         cri-tools \
         /tmp/rpms/openshift-clients-[0-9]*.rpm \
         /tmp/rpms/openshift-hyperkube-*.rpm \
-    && rpm-ostree override replace \
-         --experimental --freeze \
-         --from repo=coreos-continuous \
-         rpm-ostree rpm-ostree-libs \
+    && rpm-ostree ex rebuild \
     && rpm-ostree cleanup -m \
     && sed -i 's/^enabled=1/enabled=0/g' /etc/yum.repos.d/*.repo \
     && rm -rf /go /tmp/rpms /var/cache