ci: Fix functional tests step for pull requests in Windows GHA job

This functionality has been broken since the Windows runner image
version `20240128.1.0`.

Author: hebasto

.github/workflows/ci.yml

@@ -289,4 +289,5 @@ jobs:
       - name: Run functional tests
         env:
           TEST_RUNNER_EXTRA: ${{ github.event_name != 'pull_request' && '--extended' || '' }}
-        run: py -3 test\functional\test_runner.py --jobs $env:NUMBER_OF_PROCESSORS --ci --quiet --tmpdirprefix=$env:RUNNER_TEMP --combinedlogslen=99999999 --timeout-factor=$env:TEST_RUNNER_TIMEOUT_FACTOR $env:TEST_RUNNER_EXTRA
+        shell: cmd
+        run: py -3 test\functional\test_runner.py --jobs %NUMBER_OF_PROCESSORS% --ci --quiet --tmpdirprefix=%RUNNER_TEMP% --combinedlogslen=99999999 --timeout-factor=%TEST_RUNNER_TIMEOUT_FACTOR% %TEST_RUNNER_EXTRA%