fix(ci): use fresh clone for Windows jobs

## Problem

Windows CI jobs frequently fail during the `get_sources` phase with hundreds of `Permission denied` warnings while GitLab tries to clean read-only files from `.cache/go/pkg/mod/` left by previous builds. The failure occurs before the build script runs.

Example failed job: https://gitlab.cfdata.org/cloudflare/tun/cloudflared/-/jobs/40007572

## Fix

Set `GIT_STRATEGY: clone` on the shared `.windows-build-defaults` anchor so Windows jobs always start from a fresh clone, bypassing the broken cleanup.

## Related

- MR !1927 (affected by this issue)