Test c9s builds with CoreOS CI

An immediate benefit of the split c9s work is that we can now actually
test it in CoreOS CI (except for actually doing the derivation part).
This duplicates the Prow CI but is more reliable, easier to maintain,
and easier to inspect test results.

.cci.jenkinsfile

@@ -0,0 +1,25 @@
+// Documentation: https://github.com/coreos/coreos-ci/blob/main/README-upstream-ci.md
+
+properties([
+    // abort previous runs when a PR is updated to save resources
+    disableConcurrentBuilds(abortPrevious: true)
+])
+
+cosaPod {
+    checkout scm
+
+    cosaBuild(noStrict: true, srcConfig: env.WORKSPACE)
+
+    stage("Build ISO") {
+      parallel metal: {
+          shwrap("cd /srv/coreos && cosa buildextend-metal")
+      }, metal4k: {
+          shwrap("cd /srv/coreos && cosa buildextend-metal4k")
+      }
+      shwrap("cd /srv/coreos && cosa buildextend-live")
+    }
+
+    stage("Test ISO") {
+        kolaTestIso()
+    }
+}