ci: buildprep from latest testing-devel

That way if a PR adds a package, we'll print the pkg diff.

Author: jlebon

.cci.jenkinsfile

@@ -8,10 +8,22 @@ coreos.pod([image: "quay.io/coreos-assembler/coreos-assembler:latest", kvm: true
         dir("cosa") {
             coreos.shwrap("""
                 cosa init ${env.WORKSPACE}/fedora-coreos-config
+                # prep from the latest builds so that we generate a diff on PRs that add packages
+                cosa buildprep https://builds.coreos.fedoraproject.org/prod/streams/testing-devel/builds
                 cosa build
             """)
         }
     }
+
+    // also print the pkgdiff as a separate stage to make it more visible
+    stage("RPM Diff") {
+        dir("cosa") {
+            coreos.shwrap("""
+                jq .pkgdiff builds/latest/x86_64/meta.json
+            """)
+        }
+    }
+
     stage("Kola") {
         dir("cosa") {
             coreos.shwrap("""