Fix unused variable warning on Windows in rust/build/in_process.rs (#2471)

The `executable: bool` parameter of `install_cached_file_path` is only
read inside a `#[cfg(unix)]` block (used to chmod extracted binaries),
so it is reported as unused on non-Unix targets. This fails downstream
consumers building the crate from main with `RUSTFLAGS=-D warnings`.

Add an explicit `#[cfg(not(unix))] let _ = executable;` to mark the
parameter used on those targets, without changing Unix behavior.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: dmytrostruk

rust/build/in_process.rs

@@ -574,6 +574,12 @@ fn install_cached_file_path(
     bytes: &[u8],
     executable: bool,
 ) {
+    // `executable` only affects file permissions on Unix (see the `#[cfg(unix)]`
+    // block below); explicitly mark it used elsewhere so non-Unix targets don't
+    // warn about an unused parameter under `-D warnings`.
+    #[cfg(not(unix))]
+    let _ = executable;
+
     assert!(
         !relative_path.is_absolute()
             && !relative_path.components().any(|component| {