ci: add NuGet package cache to speed up CI

[Copilot]

Each matrix leg (net8.0, net9.0, net10.0) downloads all NuGet packages from scratch on every run. This adds actions/cache@v4 to cache ~/.nuget/packages, reducing dotnet restore time from ~30–60s to ~1–2s on cache hits.

Changes

• .github/workflows/dotnet.yml — inserts a Cache NuGet packages step between Setup .NET and Restore:

- name: Cache NuGet packages
  uses: actions/cache@v4
  with:
    path: ~/.nuget/packages
    key: ${{ runner.os }}-nuget-${{ hashFiles('**/*.csproj') }}
    restore-keys: |
      ${{ runner.os }}-nuget-

Cache key is invalidated on any .csproj change; restore key falls back to the latest same-OS cache for a partial hit. Cache is OS-scoped (not per-framework) since ~/.nuget/packages is framework-agnostic.

[NichUK]

@copilot resolve the merge conflicts in this pull request

[Copilot]

Pull request overview

This PR speeds up the repository’s GitHub Actions CI by caching the NuGet global packages folder so repeated runs don’t re-download dependencies for each matrix leg.

Changes:

• Added an actions/cache@v4 step to cache ~/.nuget/packages between actions/setup-dotnet@v4 and dotnet restore.
• Cache key is scoped by OS and invalidated when any *.csproj changes, with an OS-scoped restore key fallback.

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[Copilot]

@copilot resolve the merge conflicts in this pull request

Merged main into this branch and resolved the conflict in .github/workflows/dotnet.yml. The only clash was the cache-key prefix (nuget-${{ runner.os }}- vs ${{ runner.os }}-nuget-); I adopted the format that's already live on main (${{ runner.os }}-nuget-). Merge commit: 76ef3e8.