Add --ephemeral option to win runner (#3239)

This option needs to be hardcoded into runner start up mechanism as ARC
doesn't have proper Windows support.

b/258687270

docker/windows/runner/runner.ps1

@@ -1,2 +1,2 @@
-C:\actions-runner\config.cmd --unattended --replace --url https://github.com/${env:RUNNER_REPO} --token $env:RUNNER_TOKEN --name $env:RUNNER_NAME --work $env:RUNNER_WORKDIR --labels $env:RUNNER_LABELS;
+C:\actions-runner\config.cmd --unattended --replace --url https://github.com/${env:RUNNER_REPO} --token $env:RUNNER_TOKEN --name $env:RUNNER_NAME --work $env:RUNNER_WORKDIR --ephemeral --labels $env:RUNNER_LABELS;
 C:\actions-runner\run.cmd;