ci: use server 2019 for win builds

Author: piksel

.github/workflows/build-test.yml

@@ -117,7 +117,7 @@ jobs:
 
   Pack:
     needs: [Build, Test, CodeCov]
-    runs-on: windows-latest
+    runs-on: windows-2019
     env:
       PKG_SUFFIX: ''
       PKG_PROJ: src/ICSharpCode.SharpZipLib/ICSharpCode.SharpZipLib.csproj