ci: Write license file before build step

Add a step to write the SIXLABORS_LICENSE key to the src/sixlabors.lic file to ensure proper access to the required license during the build process.

Author: Yushu2606

.github/workflows/dotnet.yml

@@ -15,6 +15,7 @@ jobs:
         uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 10.0.x
+      - run: echo ${{ secrets.SIXLABORS_LICENSE }} > src/sixlabors.lic
       - name: Build
         run: dotnet publish -c Release -p:PublishSingleFile=true --self-contained true
       - name: Upload Artifact