ci: workflows: doc: change working directory for codecov upload

Currently the workflow is correctly uploading to codecov but all the
paths are prefixed with an extra `zephyr/`.
Use the `working-directory` parameter to ensure the paths are correct,
and also ensure that only the new.info file is ever processed.

Tested locally by doing the equivalent of what the action now does
using the codecov CLI and I was able to both reproduce the bug as well
as confirm the fix.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>