Make primary clipboard Linux only (#11843)

I guess only Linux supports the primary clipboard.

Release Notes:

- N/A

crates/gpui/src/platform/test/platform.rs

@@ -23,6 +23,7 @@ pub(crate) struct TestPlatform {
     active_display: Rc<dyn PlatformDisplay>,
     active_cursor: Mutex<CursorStyle>,
     current_clipboard_item: Mutex<Option<ClipboardItem>>,
+    #[cfg(target_os = "linux")]
     current_primary_item: Mutex<Option<ClipboardItem>>,
     pub(crate) prompts: RefCell<TestPrompts>,
     pub opened_url: RefCell<Option<String>>,
@@ -45,6 +46,7 @@ impl TestPlatform {
             active_display: Rc::new(TestDisplay::new()),
             active_window: Default::default(),
             current_clipboard_item: Mutex::new(None),
+            #[cfg(target_os = "linux")]
             current_primary_item: Mutex::new(None),
             weak: weak.clone(),
             opened_url: Default::default(),
@@ -272,6 +274,7 @@ impl Platform for TestPlatform {
         false
     }
 
+    #[cfg(target_os = "linux")]
     fn write_to_primary(&self, item: ClipboardItem) {
         *self.current_primary_item.lock() = Some(item);
     }
@@ -280,6 +283,7 @@ impl Platform for TestPlatform {
         *self.current_clipboard_item.lock() = Some(item);
     }
 
+    #[cfg(target_os = "linux")]
     fn read_from_primary(&self) -> Option<ClipboardItem> {
         self.current_primary_item.lock().clone()
     }