fix: dont prematurely shutdown runtime

[baszalmstra]

Fixes #3283

We were calling Runtime::shutdown_background which immediately shuts down the runtime without waiting for spawned work to complete.

There is a caveat:

Note however, that because we do not wait for any blocking tasks to complete, this may result in a resource leak (in that any blocking tasks are still running until they return.

When solving, we are spawning several tasks which are also driving the progress bars. Since these tasks are not properly shut down the progress bars are also not cleaned up properly.

This PR simply removes the call because I also dont think it bring us much.

[Hofer-Julian]

I cannot reproduce the problem locally, but the code change looks good to me

[ruben-arts]

It does solve the issue but it reintroduces the left over progresbar as you said.

⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
⠁ updating lock-file   [00:00:00] [────────────────────────────────────────]    0/55                                                                                                                      
Error:   × failed to solve the conda requirements of 'default' 'osx-arm64'
  ╰─▶ Cannot solve the request because of: No candidates were found for git >=123.46.0,<3.

[ruben-arts]

This actually only happens in a JetBrains terminal, both succeeding and non succeeding runs, for both the old and new pixi.

Merging this PR, thanks @baszalmstra