CI: Test with Windows Server 2019

In nearly days, `windows-latest` image on GHA was changed to Windows
Server 2022.  That causes build errors on building a cython script.

To avoid the problem, this pins the image for testing on GHA to Windows
Server 2019.

Author: tk0miya

.github/workflows/main.yml

@@ -60,7 +60,7 @@ jobs:
       if: matrix.coverage
 
   windows:
-    runs-on: windows-latest
+    runs-on: windows-2019
     strategy:
       matrix:
         architecture: [x86, x64]