ci: Use 'zephyr-gnu' toolchain variant for testing

This commit updates the CI workflow to use the `zephyr-gnu` toolchain
variant for testing the Zephyr SDK GNU toolchains.

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

Author: stephanosio

.github/workflows/ci.yml

@@ -1303,7 +1303,7 @@ jobs:
 
     env:
       SUBSET_COUNT: 3
-      ZEPHYR_TOOLCHAIN_VARIANT: zephyr
+      ZEPHYR_TOOLCHAIN_VARIANT: zephyr-gnu
 
     steps:
     - name: Set up Python