Fix Visual Studio 2019 CI build - pin to old OS

Author: claremacrae

.github/workflows/build-and-test-visual-studio-solutions.yml

@@ -32,7 +32,7 @@ jobs:
           visual-studio-2017\Release\StarterProject.exe
 
   build_vs2019_sln:
-    runs-on: windows-latest
+    runs-on: windows-2019
     steps:
       - uses: actions/checkout@v1