ci: Add SixLabors NuGet source and license file

Add the SixLabors private NuGet source before the build steps and write the license key to a file,
ensuring normal access to the required dependency packages during the build process.

Author: Yushu2606

.github/workflows/dotnet.yml

@@ -15,6 +15,8 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 10.0.x
+      - run: dotnet nuget add source https://f.feedz.io/sixlabors/sixlabors/nuget/index.json
+      - run: echo "${{ secrets.SIXLABORS_LICENSE }}" > src/sixlabors.lic
       - name: Build
         run: dotnet publish -c Release -p:PublishSingleFile=true --self-contained true
       - name: Upload Artifact