fix(complete): support the shell being specified with an absolute path

[steven-joruk]

This was failing because my SHELL is an absolute path. I know I can just SHELL=zsh ./x but I expect people would prefer to just be able to rely on their existing SHELL value.

error: invalid value '/usr/bin/zsh' for '<SHELL>'
  [possible values: bash, elvish, fish, powershell, zsh]

For more information, try '--help'.

use clap::{arg, Parser};
use clap_complete::Shell;

#[derive(Parser)]
struct Args {
    #[arg(env)]
    shell: Shell,
}

fn main() {
    Args::parse();
}

[epage]

Shell is meant to be used as the name of the shell and not to be directly compatible with $SHELL. #4447 is one change made to help when processing $SHELL.