ci: Use larger runner for Windows testing

This commit updates the CI workflow to use a larger 8-core runner for
testing Zephyr SDK distribution bundle on Windows.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/ci.yml

@@ -302,7 +302,7 @@ jobs:
         if [ "${build_host_windows_x86_64}" == "y" ]; then
           MATRIX_TESTENVS+='{
               "name": "windows-2019-x86_64",
-              "runner": "windows-2019",
+              "runner": "windows-2019-8c",
               "container": "",
               "bundle-host": "windows-x86_64",
               "bundle-archive": "zip"