Small stylistic change

src/app.rs

@@ -37,7 +37,7 @@ pub fn build_app() -> App<'static, 'static> {
             .long_help(helps[name].long)
     };
 
-    let app = App::new("fd")
+    let mut app = App::new("fd")
         .version(crate_version!())
         .usage("fd [FLAGS/OPTIONS] [<pattern>] [<path>...]")
         .setting(AppSettings::ColoredHelp)
@@ -287,16 +287,15 @@ pub fn build_app() -> App<'static, 'static> {
     // Make `--one-file-system` available only on Unix and Windows platforms, as per the
     // restrictions on the corresponding option in the `ignore` crate.
     // Provide aliases `mount` and `xdev` for people coming from `find`.
-    // It's not pretty, but I'm unaware of a way to make just part of a builder chain conditional
     if cfg!(any(unix, windows)) {
-        app.arg(
+        app = app.arg(
             arg("one-file-system")
                 .long("one-file-system")
                 .aliases(&["mount", "xdev"]),
-        )
-    } else {
-        app
+        );
     }
+
+    app
 }
 
 #[rustfmt::skip]