Revert "ci: Use Zephyr topic-sdk15 branch for testing [v2]"

This reverts commit cd04a0b.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

Author: stephanosio

.github/workflows/ci.yml

@@ -72,7 +72,7 @@ env:
   BUG_URL: 'https://github.com/zephyrproject-rtos/sdk-ng/issues'
   BUNDLE_NAME: Zephyr SDK
   BUNDLE_PREFIX: zephyr-sdk
-  ZEPHYR_REF: topic-sdk15
+  ZEPHYR_REF: main
 
 jobs:
   # Setup