CI: Use 'windows-2019' image for Windows tests

'windows-latest' is currently 'windows-2022'
with Visual Studio 2022.

Old versions of node-gyp don't know how to find Visual Studio 2022,
so the package build fails.

Switch back to 'windows-2019' with Visual Studio 2019,
which is supported by node-gyp 5.x, which apm still uses.

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     name: tests
     strategy:
       matrix:
-        os: [ubuntu-18.04, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-2019]
         channel: [beta, nightly]
       fail-fast: false
     runs-on: ${{ matrix.os }}