ci: Run tests against Zephyr v4.2-branch

SDK 0.17 needs to be tested with Zephyr v4.2.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ env:
   BUG_URL: 'https://github.com/zephyrproject-rtos/sdk-ng/issues'
   BUNDLE_NAME: Zephyr SDK
   BUNDLE_PREFIX: zephyr-sdk
-  ZEPHYR_REF: main
+  ZEPHYR_REF: v4.2-branch
 
 jobs:
   # Setup