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

···
   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 ];
   };