feat: support RUNNER_TARBALL_URL in Windows install script

[edersonbrilhante]

Description

Add support for RUNNER_TARBALL_URL as an alternative download source in the Windows runner install script (install-runner.ps1), matching the existing behavior in the Linux install-runner.sh.

When RUNNER_TARBALL_URL is set, the runner zip is downloaded directly from that URL via Invoke-WebRequest instead of from S3. If neither RUNNER_TARBALL_URL nor the S3 location is configured, the script exits with an error.

This enables Windows runners to be provisioned with a custom runner binary URL, which is useful for air-gapped environments or when using a local artifact cache.

Test Plan

• Reviewed parity with the Linux install-runner.sh logic
• Validated PowerShell syntax
• Deployed to a test environment and triggered a Windows runner workflow with RUNNER_TARBALL_URL set
• Verified fallback to S3 download when RUNNER_TARBALL_URL is unset

Related Issues

[github-actions]

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