ci: Rework to do validation after fetching git

Prep for adding Sagano as a git submodule.

.cci.jenkinsfile

@@ -7,11 +7,16 @@ cosaPod(cpus: 4, memory: "9Gi") {
     def basearch = shwrapCapture("cosa basearch")
     def mechanical_streams = ['branched', 'rawhide']
 
-    shwrap("cd config && ci/validate")
-
     shwrap("""
         mkdir -p /srv/coreos && cd /srv/coreos
         cosa init ${env.WORKSPACE}/config
+    """)
+
+    // Do basic validation here
+    shwrap("cd config && ci/validate")
+
+    shwrap("""
+        cd /srv/coreos
         python3 /usr/lib/coreos-assembler/download-overrides.py
         # prep from the latest builds so that we generate a diff on PRs that add packages
         cosa buildfetch --stream=${env.CHANGE_TARGET}