Revert "ci: Use scos (at least for now)"

This reverts commit 0b659af.

That was meant to be a temporary measure but lingered for a while. As
mentioned there, ideally, we'd build both, but if we only build one, let
it be RHCOS.

(cherry picked from commit 06ae3e1)

That should fix CI.

ci/prow-rhcos.sh

@@ -17,6 +17,6 @@ esac
 export COSA_SKIP_OVERLAY=1
 # Create a temporary cosa workdir
 cd "$(mktemp -d)"
-cosa init --variant scos --transient -b "${RHCOS_BRANCH}" https://github.com/openshift/os
+cosa init --transient -b "${RHCOS_BRANCH}" https://github.com/openshift/os
 # Use a COSA specifc test entry point to focus on tests relevant for COSA
 exec src/config/ci/prow-entrypoint.sh rhcos-cosa-prow-pr-ci