ci: run Debian verifier with Bash

Why:
The Debian container executes workflow run steps with sh by default. The
published-package verifier used Bash pipeline status syntax after its smoke
tests, so a successful install was reported as a workflow failure.

Impact:
Published Debian packages can complete the end-to-end verification gate,
while genuine install or smoke-test failures still fail the workflow.

Before/After:
Before: the exact package installed and validated, then sh rejected a Bash
array assignment.
After: the verifier explicitly uses Bash and records the pipefail result as
a scalar status.

Technical Overview:
Set the verifier job run shell to Bash and preserve the pipeline result
without an unnecessary array.

With the help of AI-Agents: Codex

.github/workflows/debian_daily_stable.yml

@@ -536,6 +536,9 @@ jobs:
     container:
       image: debian:trixie
       options: --user root
+    defaults:
+      run:
+        shell: bash
     environment: debian-daily-stable
     permissions:
       contents: read
@@ -626,11 +629,11 @@ jobs:
             rsyslogd -v
             rsyslogd -N1
           ) 2>&1 | tee .ci/flake-evidence/logs/debian-published-install.log
-          install_status=("${PIPESTATUS[@]}")
+          install_status=$?
           set -e
           devtools/ci-flake-phase.sh end \
-            debian-published-install custom "${install_status[0]}"
-          exit "${install_status[0]}"
+            debian-published-install custom "$install_status"
+          exit "$install_status"
 
       - name: Upload publication failure evidence
         if: >-