ci: address self-review findings

- Wire up warm-bench: set BENCH_CACHE_DIR on the setup job so the bench-base
  cache actually activates (was inert with no dir set, so every run did a
  full bench init). Lives on the node-local bench-staging hostPath; any
  miss/failure still falls back to a full init.
- run_ci_step: capture the timeout exit code under `set -e` (the previous
  `timeout ...` + `ec=$?` aborted at the timeout line on failure, skipping
  ::endgroup:: and the exit-code return).
- Raise the per-step timeout 600s -> 1800s so a contended reinstall isn't
  killed before the 40-min job timeout.
- Propagate DB through the su re-exec in start-db.sh / hydrate.sh so a
  DB=postgres invocation can't silently fall back to the mariadb branch.
- Simplify the coverage job `if` to the equivalent plain non-PR gate.

.github/helper/start-db.sh

@@ -19,7 +19,7 @@ ci_user="${ERPNEXT_CI_USER:-frappe}"
 # refused anyway). Mirrors install.sh's user switch.
 if [ "$(id -u)" = "0" ] && [ "${SKIP_SYSTEM_SETUP:-0}" = "1" ] && [ "$ci_user" != "root" ]; then
     exec su -m "$ci_user" -s /bin/bash -c \
-        "ERPNEXT_CI_USER='$ci_user' CI_DB_DATADIR='${CI_DB_DATADIR:-}' bash '$0'"
+        "ERPNEXT_CI_USER='$ci_user' CI_DB_DATADIR='${CI_DB_DATADIR:-}' DB='${DB:-}' bash '$0'"
 fi
 
 # --- PostgreSQL (GitHub-hosted CI): run in-runner on a PGDATA so it bakes into the artifact,