deploy-cpp.yml: apt-get install nuget fails on ubuntu-24.04, and publishRelease still creates the release #6

Summary

deploy-cpp.yml (at 06a7067) installs the NuGet CLI with sudo apt-get install nuget. ubuntu-latest is Ubuntu 24.04, and Ubuntu 24.04 no longer ships that package, so pushToNuget fails with exit code 100. The publishRelease job does not depend on pushToNuget, so it still creates the GitHub release. The result is a release page that links to a NuGet package that was never published.

Evidence

From linksplatform/Interfaces run 36213155523 (image ubuntu24/20260920.314):

main / pushToNuget  Package nuget is not available, but is referred to by another package.
main / pushToNuget  E: Package 'nuget' has no installation candidate
main / pushToNuget  ##[error]Process completed with exit code 100.

The same run still created release cpp_0.4.1 at 02:55:28Z. The releases cpp_0.3.43 and cpp_0.4.0 were created the same way. Meanwhile nuget.org stops at 0.3.41.

Minimal reproduction

docker run --rm ubuntu:24.04 sh -c 'apt-get update -qq && apt-cache policy nuget && apt-get install -y nuget'
# nuget:
#   Installed: (none)
#   Candidate: (none)
# E: Unable to locate package nuget      (exit 100)

Workaround

linksplatform/Interfaces#151 stops calling the reusable workflow and does the following instead:

• packs the .nuspec with dotnet pack, which the runner image already has, through a tiny project that sets NuspecFile, NuspecBasePath and NoBuild;
• uploads the .nupkg as an artifact from the test workflow;
• pushes with dotnet nuget push --skip-duplicate;
• creates the release with needs: [test, pushToNuget], so the release is never created without the package.

The scripts involved are pack-cpp-nuget.mjs and push-nuget-package.sh.

Suggested fix

1. Replace apt-get install nuget and nuget pack / nuget push with dotnet pack (or NuGet/setup-nuget pinned by commit) and dotnet nuget push --skip-duplicate.
2. Add pushToNuget to publishRelease.needs. Do not create the release when the push failed.
3. Before pushing, check whether the version is already on nuget.org (https://api.nuget.org/v3-flatcontainer/<id>/index.json). This keeps a re-run idempotent.
4. Pin runs-on: ubuntu-24.04, so a future image migration fails in a PR instead of in production.