Windows Sandbox: treat <workspace_root>/.git as read-only in workspace-write mode (#7142)

this functionality is
[supported](https://github.com/openai/codex/blob/main/codex-rs/protocol/src/protocol.rs#L421-L422)
in the MacOs sandbox as well. Adding it to Windows for parity

This PR also changes `rust-ci.yaml` to work around a github `hashFiles`
issue. Others have done something
[similar](https://github.com/openai/superassistant/pull/32156) today

codex-rs/Cargo.lock

@@ -1646,6 +1646,7 @@ dependencies = [
  "rand 0.8.5",
  "serde",
  "serde_json",
+ "tempfile",
  "windows-sys 0.52.0",
 ]