ci: Update to use new buildroot pod

Matching coreos/ignition#1182
and discussion in coreos/fedora-coreos-tracker#764

.cci.jenkinsfile

@@ -3,7 +3,7 @@
 stage("Build") {
 parallel rpms: {
   def n = 5
-  cosaPod(buildroot: true, runAsUser: 0, memory: "2Gi", cpu: "${n}") {
+  buildPod(memory: "2Gi", cpu: "${n}") {
       checkout scm
       // 2:1 job to CPU at most should keep us from getting kicked out
       shwrap("""RPM_BUILD_NCPUS=${n} CARGO_BUILD_JOBS=${n} ./ci/coreosci-rpmbuild.sh
@@ -15,7 +15,7 @@ parallel rpms: {
   }
 },
 codestyle: {
-  cosaPod(buildroot: true) {
+  buildPod {
       checkout scm
       shwrap("""
         # Ensures that we get refs to aid `git describe`