Limit run-external --redirect-combine sh test to not(Windows) (#10905)

# Description

Limit the test `-p nu-command --test main
commands::run_external::redirect_combine` which uses `sh` to running on
`not(Windows)` like is done for other tests assuming unixy CLI items;
`sh` doesn't exist on Windows.

# User-Facing Changes

None; this is a change to tests only.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-command/tests/commands/run_external.rs

@@ -321,6 +321,7 @@ fn quotes_trimmed_when_shelling_out() {
     assert_eq!(actual.out, "foo");
 }
 
+#[cfg(not(windows))]
 #[test]
 fn redirect_combine() {
     Playground::setup("redirect_combine", |dirs, _| {