ci: check first boot log from PR build, not base image

The ostree test checks journalctl -b 1 for the first boot message,
but boot 1 uses the base image's greenboot (before rpm-ostree override
replace). Change to -b 2 which is the first boot after the PR build
is installed.

Assisted-by: Claude (claude-opus-4-6)
Signed-off-by: Paul Whalen <pwhalen@fedoraproject.org>

Author: Paul Whalen

tests/greenboot-ostree.yaml

@@ -71,11 +71,11 @@
           set_fact:
             failed_counter: "{{ failed_counter | int + 1 }}"
 
-    # case: check proper first boot handling
-    - name: check greenboot-healthcheck handles first boot correctly
+    # case: check proper first boot handling after greenboot override
+    - name: check greenboot-healthcheck handles first boot with PR build correctly
       block:
-        - name: get greenboot-healthcheck log from first boot
-          command: "journalctl -b 1 -u greenboot-healthcheck.service --no-pager"
+        - name: get greenboot-healthcheck log from first boot with PR build
+          command: "journalctl -b 2 -u greenboot-healthcheck.service --no-pager"
           become: yes
           register: result_healthcheck_first_boot