This reverts commit fe90d5a666a4d649ddb35bf6d617bf5e98be777c.

Without the change `passthru` override deleted `fetchGitHashesScript`
entry and caused updater scripts to fail:

```
$ nix-instantiate -A butterfly.updateScript
error:
       … while evaluating the attribute 'command'
         at pkgs/common-updater/combinators.nix:190:7:
          189|     {
          190|       command = commandsToShellInvocation (map ({ command, ... }: command) scripts);
             |       ^
          191|       supportedFeatures =

       … while evaluating the attribute 'paths'
         at pkgs/common-updater/combinators.nix:91:7:
           90|       commands = commands ++ [ new.args ];
           91|       paths = paths ++ new.paths;
             |       ^
           92|       maxArgIndex = new.maxArgIndex;

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: attribute 'fetchGitHashesScript' missing
       at pkgs/by-name/bu/butterfly/package.nix:64:11:
           63|         command = [
           64|           dart.fetchGitHashesScript
             |           ^
           65|           "--input"
```

Let's use attribute merge instead.