Fix Build CI: fetch libilbc submodule before Docker build

[PierreFouquet]

Problem

The Build workflow (buildimage.yaml) has been failing on every push to main for weeks. The Docker build dies at the cmake step:

CMake Error: The source directory "/src/libilbc" does not appear to contain CMakeLists.txt.
...
ld: cannot find -l:libilbc.so.3: No such file or directory
error: could not compile `projectrtp` (lib) due to 1 previous error

Root cause

libilbc is a git submodule (.gitmodules: path = libilbc, url = https://github.com/TimothyGu/libilbc). The Dockerfile does COPY libilbc/ /src/libilbc/ and runs cmake on it to build libilbc.so.3.

The checkout step used actions/checkout@v5 with no submodules: setting. actions/checkout does not fetch submodules by default (any version), so libilbc/ is empty in the build context → cmake finds no CMakeLists.txt → libilbc.so.3 is never built → the Rust cdylib fails to link.

Fix

Add submodules: recursive to the checkout step so the build context contains the libilbc source. rust.yml already does this. The submodule is a public repo, so no extra token is needed.

This is independent of the rustfmt drift failing the Rust CI job (handled in a separate PR), and is not related to the action-version-bump PRs — checkout@v5 and @v6 both skip submodules by default.

🤖 Generated with Claude Code