switch greetd wayland session manager from niri to cage fork

pkgs/cage.nix

@@ -0,0 +1,41 @@
+{
+  pkgs,
+  srcs,
+  ...
+}: let
+  inherit (srcs) cage;
+  cageWithEnvOption = pkgs.cage.overrideAttrs (final: prev: {
+    inherit (cage) src version;
+  });
+in
+  pkgs.writeShellApplication {
+    name = "cage";
+    runtimeInputs = [cageWithEnvOption];
+    text = ''
+      args=()
+      output=
+
+      while [[ $# -gt 0 ]]; do
+        case "$1" in
+          -o)
+            if [[ $# -ge 2 ]]; then
+              output="$2"
+              shift 2
+            else
+              args+=("$1")
+              shift
+            fi
+            ;;
+          *)
+            args+=("$1")
+            shift
+            ;;
+        esac
+      done
+      if [[ -n "$output" ]]; then
+        CAGE_OUTPUT_NAME="$output" cage "''${args[@]}"
+      else
+        cage "''${args[@]}"
+      fi
+    '';
+  }