pop-launcher: mark linux-only

Based on:
```
   Compiling async-pidfd v0.1.4
error: pidfd only works on Linux
   --> /private/tmp/nix-build-pop-launcher-1.2.1.drv-0/pop-launcher-1.2.1-vendor.tar.gz/async-pidfd/src/lib.rs:106:1
    |
106 | compile_error!("pidfd only works on Linux");
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

pkgs/applications/misc/pop-launcher/default.nix

@@ -54,6 +54,7 @@ rustPlatform.buildRustPackage rec {
   meta = with lib; {
     description = "Modular IPC-based desktop launcher service";
     homepage = "https://github.com/pop-os/launcher";
+    platforms = platforms.linux;
     license = licenses.mpl20;
     maintainers = with maintainers; [ samhug ];
   };