ci: add NuGet package caching to test job

Adds actions/cache@v4 for ~/.nuget/packages keyed on csproj hashes.
Saves ~30-60s on subsequent CI runs by reusing cached packages.

Uses csproj hash (not lock files) since this repo doesn't use
packages.lock.json, which is why setup-dotnet's cache: true won't work.

Based on Repo Assist suggestion (#66).

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

.github/workflows/ci.yml

@@ -20,6 +20,13 @@ jobs:
       with:
         dotnet-version: 10.0.x
 
+    - name: Cache NuGet packages
+      uses: actions/cache@v4
+      with:
+        path: ~/.nuget/packages
+        key: nuget-${{ runner.os }}-${{ hashFiles('**/*.csproj') }}
+        restore-keys: nuget-${{ runner.os }}-
+
     - name: Install GitVersion
       uses: gittools/actions/gitversion/setup@v4
       with: