GHA: Explicitly install nuget

While nuget is preinstalled on GitHub Actions provided Windows images
via Chocolatey, it may not be available on custom images.

Author: maruel-tbc

.github/workflows/swift-toolchain.yml

@@ -646,6 +646,9 @@ jobs:
         with:
           python-version: '${{ env.PYTHON_VERSION }}'
 
+      - uses: nuget/setup-nuget@v2
+        if: matrix.arch == 'arm64'
+
       # TODO(lxbndr) use actions/cache to improve this step timings
       - name: Install Python ${{ env.PYTHON_VERSION }} (arm64)
         if: matrix.arch == 'arm64'