add(system): btop.nix

gives more permission to btop (specifically for reading CPU power)

modules/nixos/btop.nix

@@ -0,0 +1,15 @@
+{ pkgs, ... }: {
+  # Install btop system-wide with ROCm support for GPU monitoring.
+  environment.systemPackages = [
+    (pkgs.btop.override { rocmSupport = true; })
+  ];
+
+  # Create a security wrapper for btop to grant it capabilities
+  # to read CPU power information without running as root.
+  security.wrappers."btop-power" = {
+    source = "${pkgs.btop.override { rocmSupport = true; }}/bin/btop";
+    owner = "root";
+    group = "root";
+    capabilities = "cap_dac_read_search,cap_sys_ptrace+ep";
+  };
+}