[github-actions] Use windows-2019 instead of windows-latest for now

Built is failing because of some python mess
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 4d5f711..5108034 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -96,7 +96,7 @@
         name: libplist-latest_macOS
         path: libplist.tar
   build-windows:
-    runs-on: windows-latest
+    runs-on: windows-2019
     defaults:
       run:
         shell: msys2 {0}