Make zinit and zi use parallel updates

[domglusk]

Standards checklist:

• [Y] The PR title is descriptive.
• [Y] I have read CONTRIBUTING.md
• [Y] The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• [Y] The code passes tests (cargo test)
• [Y] Optional: I have tested the code myself

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[SteveLauC]

You need to run:

$ cd topgrade
$ cargo fmt

to format the code

[domglusk]

Sorry it's my first time doing a pull request, it was giving me a cargo fmt isn't a command but got it working

[SteveLauC]

Sorry it's my first time doing a pull request

Then congratulations! you did it! :)

Thanks!