ci: twister: Do not run on third-party repositories

This commit updates the twister workflow to only run on the
repositories in the zephyrproject-rtos organisation because this
workflow requires a custom runner type only available within the
zephyrproject-rtos organisation.

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

.github/workflows/twister.yaml

@@ -19,6 +19,7 @@ concurrency:
 
 jobs:
   twister-build-prep:
+    if: github.repository_owner == 'zephyrproject-rtos'
     runs-on: zephyr-runner-linux-x64-4xlarge
     container:
       image: ghcr.io/zephyrproject-rtos/ci:v0.24.5