test: build apisix-runtime against apisix-nginx-module#127 (do not merge)

[AlinsRan]

Do not merge. This PR only uses CI to package an apisix-runtime deb for verifying api7/apisix-nginx-module#127 before it is tagged.

• build-apisix-runtime.sh: apisix_nginx_module_ver points at stream-metrics-session-tag-build, a slash-free copy of the CI: add label to control CI #127 branch (the module directory is named after the ref, so a / breaks patch.sh).
• package-apisix-runtime-deb-ubuntu20.04.yml: builds on debian:bookworm-slim instead of ubuntu:24.04, the same base the release packages use, so the artifact runs on older glibc hosts.

Once #127 is merged and tagged, the real bump goes through a normal chore: bump apisix-nginx-module PR, and this one gets closed.

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