ci: clang: Clone cached Zephyr repository

This commit updates the clang workflow to pre-clone the Zephyr
repository from the runner repository cache.

Note that the `origin` remote URL is reconfigured to that of the GitHub
Zephyr repository because the checkout action attempts to delete
everything and re-clone otherwise.

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