fix: override meta.mainProgram on custom appName

nix/mkNeovim.nix

@@ -207,4 +207,8 @@ with lib;
         + lib.optionalString isCustomAppName ''
           mv $out/bin/nvim $out/bin/${lib.escapeShellArg appName}
         '';
+      meta.mainProgram 
+        = if isCustomAppName 
+            then appName 
+            else oa.meta.mainProgram;
     })