ci: build metal images in one OSBuild invocation

We can do this all with OSBuild now so let's not run parallel
processes here. This will enable us to drop the cache snapshot
bits since we won't have two processes trying to use the cache
qcow at the same time.

.cci.jenkinsfile

@@ -30,7 +30,7 @@ pod(image: imageName + ":latest", kvm: true, cpu: "${cpuCount}", memory: "${memo
     kola(cosaDir: "/srv", addExtTests: ["${env.WORKSPACE}/ci/run-kola-self-tests"])
 
     stage("Build Metal") {
-        cosaParallelCmds(cosaDir: "/srv", commands: ["metal", "metal4k"])
+        utils.cosaCmd(cosaDir: "/srv", args: "osbuild metal metal4k")
     }
 
     stage("Build Live Images") {