Switch build workflow to windows-latest.

Author: tymorrow

.github/workflows/build.yml

@@ -2,7 +2,7 @@ name: Sphinx Build
 on: push
 jobs:
   build:
-    runs-on: windows-2019
+    runs-on: windows-latest
     steps:
     - uses: actions/checkout@v2
     - name: Setup MSBuild