fix(test): skip emulation artifact roundtrip test on Windows

The artifact roundtrip test runs `bash -c "mkdir ... && echo ..."`
through EmulationRuntime, which does `Command::new("bash")`. On
Windows CI runners, this resolves to WSL's `bash.exe` sitting in
`C:\Windows\System32\`, not Git Bash. And since the runner has no
WSL distributions installed, the test blows up with a helpful
"Windows Subsystem for Linux has no installed distributions" and
a link to the Microsoft Store.

The test is inherently Unix-only — it exercises Unix shell commands
on a real EmulationRuntime. Gate it with `cfg(not(target_os =
"windows"))`.

crates/executor/src/engine.rs

@@ -7630,6 +7630,7 @@ runs:
     /// This test drives a real `EmulationRuntime` end-to-end through
     /// run → upload-artifact → download-artifact and asserts the payload
     /// round-trips byte-for-byte.
+    #[cfg(not(target_os = "windows"))]
     #[tokio::test]
     async fn run_step_upload_download_artifact_roundtrip_emulation() {
         let runtime = emulation::EmulationRuntime::new();