Do not stream bundle exec rake -P output

This output is meant to be machine readable only, it should be timed and not streamed.

I.e. we want the output to be

```
- Rake assets install
  - Rake detected (`rake` gem found, `Rakefile` found ad `/workspace/Rakefile`)
  - Running `bundle exec rake -P --trace` .... (2.1s)
  - Compiling assets with cache (detected `rake assets:precompile` and `rake assets:clean` via `bundle exec rake -P`)
```

And not

```
- Rake assets install
  - Rake detected (`rake` gem found, `Rakefile` found ad `/workspace/Rakefile`)
  - Running `bundle exec rake -P --trace`

      rake about
          environment
      rake action_mailbox:ingress:environment
      rake action_mailbox:ingress:exim
          action_mailbox:ingress:environment
      rake action_mailbox:ingress:postfix
          action_mailbox:ingress:environment
      rake action_mailbox:ingress:qmail
          action_mailbox:ingress:environment
      rake action_mailbox:install
      rake action_mailbox:install:migrations
      rake action_text:install
      rake action_text:install:migrations
      rake active_storage:install
          environment
      rake active_storage:install:migrations
      rake active_storage:update

	  [...]

  - Done (2.111s)
  - Compiling assets with cache (detected `rake assets:precompile` and `rake assets:clean` via `bundle exec rake -P`)

```

buildpacks/ruby/src/gem_list.rs

@@ -67,11 +67,11 @@ impl GemList {
         let mut cmd = Command::new("bundle");
         cmd.arg("list").env_clear().envs(envs);
 
-        log_step_stream(format!("Running {}", fmt::command(cmd.name())), |stream| {
-            let output = cmd.stream_output(stream.io(), stream.io())?;
+        let output = log_step_timed(format!("Running {}", fmt::command(cmd.name())), || {
+            cmd.named_output()
+        })?;
 
-            GemList::from_str(&output.stdout_lossy())
-        })
+        GemList::from_str(&output.stdout_lossy())
     }
 
     #[must_use]