ci: Explicitly request windows-2019

'windows-latest' has been bumped by GitHub to windows-2022 causing
breakage. First priority is to get CI green again and fix build
with 6.9

For issue #85

Author: iamsergio

.github/workflows/build.yml

@@ -20,7 +20,7 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-          - windows-latest
+          - windows-2019
           - macos-latest
         build_type:
           - Debug