Fix CI: host-mode runner has no Node.js for actions/checkout

Host-mode jobs run directly in the runner container instead of a
spawned per-job container, so JS actions like actions/checkout@v4
have no Node.js runtime to execute with. Replace it with a plain git
fetch/checkout in the deploy job.

docs/deployment.adoc

@@ -112,6 +112,11 @@ of Puma's bind address.
 `.gitea/workflows/ci.yml`'s `deploy` job runs after `check` passes, only
 on a push to `main`, with `runs-on: host`:
 
+. Checks out the repo with plain `git` (`fetch` + `checkout`,
+  authenticated via a one-off `http.extraHeader`), not
+  `actions/checkout@v4` — that's a JS action, and host-mode jobs run
+  directly in the runner container, which has `git` but no Node.js
+  runtime to execute JS actions with.
 . Copies the checkout into `/srv/docker/maptool/` using plain
   `cp`/`rm` (no `rsync` in the runner image): clears everything in the
   target except `data/`, copies the fresh checkout over, then drops