build and build-msix jobs were running dotnet restore with no caching, downloading packages fresh on every run. The test job already uses actions/cache@v4 with a nuget-{os}-{csproj-hash} key — adding the same step to the other two jobs lets all matrix runners share the cache.

Changes

• build job: added actions/cache@v4 before dotnet restore
• build-msix job: added actions/cache@v4 before dotnet restore

Cache configuration (identical to test job):

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

Cache is keyed on .csproj content hash, so stale cache on dependency changes is handled automatically.