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/workflows/server-tests-mariadb.yml

@@ -106,6 +106,7 @@ jobs:
           WKHTMLTOX_DEB: /tmp/wkhtmltox.deb
           SKIP_SYSTEM_SETUP: "1"
           SKIP_WKHTMLTOX_SETUP: "1"
+          BENCH_CACHE_DIR: /opt/ci-bench-staging/base-cache
 
       # Clean shutdown (consistent InnoDB datadir), then stage it inside the bench for packaging.
       - name: Stop DB and stage datadir
@@ -201,7 +202,7 @@ jobs:
   coverage:
     name: Coverage Wrap Up
     needs: [test]
-    if: ${{ always() && github.event_name != 'pull_request' && !cancelled() && !failure() }}
+    if: ${{ github.event_name != 'pull_request' }}
     runs-on: ubuntu-latest
     steps:
       - name: Clone