set windows version to 2019 on CI

Author: Alberto Iannaccone

.github/workflows/build.yml

@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         config:
-          - os: windows-latest
+          - os: windows-2019
           - os: ubuntu-18.04 # https://github.com/arduino/arduino-ide/issues/259
           - os: macos-latest
     runs-on: ${{ matrix.config.os }}