ci: cache NuGet packages to speed up builds

Add actions/cache@v4 to cache the ~/.nuget/packages directory,
keyed on paket.lock hash. This avoids re-downloading packages on
every CI run, which should significantly reduce build times on
cache hits.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

.github/workflows/dotnet.yml

@@ -39,6 +39,13 @@ jobs:
       uses: actions/setup-dotnet@v5
       with:
         dotnet-version: ${{ matrix.dotnet }}
+    - name: Cache NuGet packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.nuget/packages
+        key: ${{ runner.os }}-nuget-${{ hashFiles('**/paket.lock') }}
+        restore-keys: |
+          ${{ runner.os }}-nuget-
     - name: Install local tools
       run: dotnet tool restore
     - name: Paket Restore