Avoids tying CI to a personal/bot Gitea account: the same DEPLOY_SSH_KEY
secret used for the webserver rsync step is reused as a read-only Gitea
deploy key on hmk/pelican-latex. The submodule's tracked HTTPS URL is
overridden to SSH just for this checkout so it authenticates with that
key instead of needing a token with access to both repos.
The default per-job Actions token only covers the triggering repo; the
theme submodule lives in a separate private repo (hmk/pelican-latex), so
cloning it needs a token with read access to both. Add CHECKOUT_TOKEN
as a repo secret (a Gitea access token with repo read scope) for this
to work.
haemka