chore: bump apisix-nginx-module to 1.19.4

[jarvis9443]

Bump apisix-nginx-module from 1.19.3 to 1.19.4.

What's new in 1.19.4

Adds upstream state FFI functions (push_upstream_state / update_upstream_state) that allow Lua code to populate nginx upstream variables (`$upstream_status`, `$upstream_addr`, `$upstream_response_time`, `$upstream_header_time`, `$upstream_connect_time`, `$upstream_response_length`) when the upstream module is bypassed by cosocket-based transports (e.g., ai-proxy plugin).

See: api7/apisix-nginx-module#112

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[Copilot]

Pull request overview

Bumps the apisix-nginx-module dependency used by the runtime build script from 1.19.3 to 1.19.4 to pick up newly added upstream state FFI functions.

Changes:

• Update apisix_nginx_module_ver to 1.19.4 in the runtime build script.

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