enhance: test on Windows and fix what it found (#1348)

Adds a weekly and on-demand workflow that runs the cross-compiled tests on Windows Server 2025 Core in a [dockur/windows](https://github.com/dockur/windows) QEMU VM, and fixes the Windows bugs it found in host and UNC paths, env handling, shell quoting, post-task scripts, health checks, checkout, action symlinks, step summaries, Git client certificates and docker mode from a Windows host.

Host-mode steps now emit UTF-8, `bash` falls back to Git's bash instead of the WSL launcher, the runner can run as a native Windows service, steps can keep background helpers with `RUNNER_TRACKING_ID`, and Windows arm64 binaries are released.

Fixes: https://gitea.com/gitea/runner/issues/1339
Fixes: https://gitea.com/gitea/runner/issues/637
Fixes: https://gitea.com/gitea/runner/issues/1172
Fixes: https://gitea.com/gitea/runner/issues/347
Reviewed-on: https://gitea.com/gitea/runner/pulls/1348
Reviewed-by: bircni <[email protected]>

DEVELOPMENT.md

@@ -69,3 +69,12 @@ Docker and Kubernetes.
 
 The suite shares one Gitea, regular runner and k3s cluster. Cache and ephemeral scenarios use isolated
 repository runners. A run path is `<workflow>@<ref>` and a log row is `<timestamp>Z <payload>`.
+
+## Windows tests
+
+`make test-windows` cross-compiles the test binaries, installs Windows Server 2025 Core in a
+[dockur/windows](https://github.com/dockur/windows) QEMU VM on the docker host and runs them there.
+The host must be amd64 and allow privileged containers. `WINDOWS_TEST_TIMEOUT` sets the minutes
+allowed for installing Windows and running the tests, 165 by default. The VM uses KVM when the host
+has `/dev/kvm`, otherwise Windows is emulated and much slower. Docker-backed tests skip in the VM
+and the race detector is off, it needs cgo. CI runs this weekly and on demand.