GitHub: Explicitly use MSVC 2019 machines

windows-latest is "transitioning to 2022" currently, and that seems to
mean that for some users this is already 2022 and for some still 2019.
Explicitly state something.

Change-Id: I1e7ba6fa60b9fe44c78505b322ff8278c2e41ee6
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

.github/workflows/build_cmake.yml

@@ -25,8 +25,8 @@ jobs:
       matrix:
         config:
         - {
-            name: "Windows Latest MSVC", artifact: "Windows-MSVC",
-            os: windows-latest,
+            name: "Windows MSVC 2019", artifact: "Windows-MSVC",
+            os: windows-2019,
             cc: "cl", cxx: "cl",
             environment_script: "C:/Program Files (x86)/Microsoft Visual Studio/2019/Enterprise/VC/Auxiliary/Build/vcvars64.bat",
             is_msvc: true