[pkgs] planka: small tweaks

packages/planka/package.nix

···
 in
 stdenv.mkDerivation (finalAttrs: {
   pname = "planka";
-  inherit version src meta;
+  inherit version src;
 
   sourceRoot = "${finalAttrs.src.name}/server";
 
···
   '';
 
   passthru.updateScript = nix-update-script { extraArgs = [ "--version=unstable" ]; };
+  meta = meta // {
+    mainProgram = "planka";
+  };
 })