Fix lint

[willbush]

I noticed this lint when going through the https://github.com/rust-lang/rustup/blob/master/CONTRIBUTING.md and running the suggested clippy command.

[nix-shell:~/code/rustup]$ cargo --version
cargo 1.55.0-nightly (9233aa06c 2021-06-22)
[nix-shell:~/code/rustup]$ cargo clippy --all --all-features --all-targets -- -D warnings

... elided output

error: calls to `push` immediately after creation
   --> tests/mock/clitools.rs:626:5
    |
626 | /     let mut arg_strings: Vec<Box<str>> = Vec::new();
627 | |     arg_strings.push(name.to_owned().into_boxed_str());
    | |_______________________________________________________^ help: consider using the `vec![]` macro: `let mut arg_strings: Vec<Box<str>> = vec![..];`
    |
    = note: `-D clippy::vec-init-then-push` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#vec_init_then_push

[0xPoe]

Thanks for your contribution!

See: rust-lang/rust-clippy#7071

Currently we consider it a false positive.