CI: change msvc2019* runners to windows-2019

Github updated their runners. Windows-latest is now based on Windows
Server 2022 and comes with different tool versions.
MSVC 2019 is now available via the windows-2019 runner.

.github/workflows/windows.yml

@@ -65,7 +65,7 @@ jobs:
       run: cd build ; ctest -j 10 -C Release --output-on-failure
 
   msvc2019:
-    runs-on: windows-latest
+    runs-on: windows-2019
     strategy:
       matrix:
         build_type: [Debug, Release]
@@ -85,7 +85,7 @@ jobs:
       run: cd build ; ctest -j 10 -C ${{ matrix.build_type }} --output-on-failure
 
   msvc2019_latest:
-    runs-on: windows-latest
+    runs-on: windows-2019
 
     steps:
     - uses: actions/checkout@v2