image-tests: Update to recent Yocto and Isar revisions

This moves poky to 4.0.3 and Isar to the next revision of the day. The
latter is needed as we have no release with sbuild support yet.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

image-tests/poky/kas.yml

@@ -30,7 +30,7 @@ target: zlib-native
 repos:
   poky:
     url: https://git.yoctoproject.org/poky.git
-    refspec: b53230c08d9f02ecaf35b4f0b70512abbf10ae11
+    refspec: 387ab5f18b17c3af3e9e30dc58584641a70f359f
     layers:
       meta:
       meta-poky: