lint, build, unit and mutation each restored packages from the network on every
run. There are no lock files (so setup-dotnet's built-in cache doesn't apply), so
cache ~/.nuget/packages keyed on the project files via actions/cache. Pinned @v3
to avoid the GHES guard that breaks @v4 on Gitea (gitea-actions-gotchas.md); the
cache is best-effort, so a miss simply restores from the network.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>