CI: Include submodules in the release source archive #3445

Since cuda-core 1.3.0, building cuda_core from source requires the CCCL submodule at cuda_core/third_party/cccl: _get_cccl_include_dirs() treats a missing submodule as a build error, by design. The source archive attached to each release is a plain git archive (release-upload.yml), which leaves submodules empty, so that archive alone can no longer build cuda-core.

Downstream packagers now have to fetch CCCL separately and hard-code the submodule commit, as the conda-forge recipe does in conda-forge/cuda-core-feedstock#34. That pin has to be kept in sync by hand: the conda-forge bot bumps the version and re-hashes the sources, but the CCCL URL doesn't depend on the version, so its version-bump PRs would keep building against the old CCCL commit.

Proposal: check out with submodules: true and append each submodule to the tarball, e.g.:

export OUT="${PWD}/release"
git archive --format=tar --prefix="${ARCHIVE_NAME}/" -o "${OUT}/${ARCHIVE_NAME}.tar" "${GIT_TAG}"
git submodule foreach --quiet --recursive '
  git archive --format=tar --prefix="${ARCHIVE_NAME}/${displaypath}/" -o "${OUT}/sub.tar" HEAD
  tar -Af "${OUT}/${ARCHIVE_NAME}.tar" "${OUT}/sub.tar" && rm "${OUT}/sub.tar"'
gzip -n "${OUT}/${ARCHIVE_NAME}.tar"

The result is byte-for-byte reproducible. At cuda-core-v1.3.0, the archive grows from 6.0 MB to 19.0 MB, or to 7.4 MB if only libcudacxx/include and the licenses are included. The 1.3.0 asset stays as is; once a release ships the submodule, the feedstock can go back to a single source.

Related: #2715

-- Leo's bot