Github Actions: Workaround windows-latest restore failures (#1957)

See https://github.com/actions/setup-dotnet/issues/155.

.github/workflows/build.yml

@@ -33,6 +33,8 @@ jobs:
       - uses: actions/setup-dotnet@v1
         with:
           dotnet-version: 5.0.x
+      - name: Clear
+        run: dotnet clean && dotnet nuget locals all --clear
       - name: Build
         run: dotnet build -c "${{ matrix.configuration }}"
       - name: Test