dart-source: don't clobber passthru elements

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.

pkgs/development/compilers/dart/source/default.nix

@@ -247,9 +247,11 @@ dart-bin.overrideAttrs (oldAttrs: {
     runHook postInstall
   '';
 
-  passthru.updateScript = writeShellScript "update-dart" ''
-    ${lib.getExe nix-update} --version=$(${lib.getExe curlMinimal} --fail --location --silent https://storage.googleapis.com/dart-archive/channels/stable/release/latest/VERSION | ${lib.getExe jq} --raw-output .version)
-  '';
+  passthru = oldAttrs.passthru // {
+    updateScript = writeShellScript "update-dart" ''
+      ${lib.getExe nix-update} --version=$(${lib.getExe curlMinimal} --fail --location --silent https://storage.googleapis.com/dart-archive/channels/stable/release/latest/VERSION | ${lib.getExe jq} --raw-output .version)
+    '';
+  };
 
   meta = oldAttrs.meta // {
     platforms = [