Does not properly detect cross-platform PowerShell (pwsh)

### Please complete the following tasks - [x] I have searched the [discussions](https://github.com/clap-rs/clap/discussions) - [x] I have searched the [open](https://github.com/clap-rs/clap/issues) and [rejected](https://github.com/clap-rs/clap/issues?q=is%3Aissue+label%3AS-wont-fix+is%3Aclosed) issues ### Rust Version rustc 1.98.0-nightly (beae78130 2026-06-09) ### Clap Version 4.6.1 ### Minimal reproducible code ```rust fn main() { clap_complete::CompleteEnv::with_factory(Args::command).complete(); } #[derive(clap::Parser)] #[command] struct Args; ``` ### Steps to reproduce the bug with the above code 1. Install pwsh e.g., `brew install powershell` on macOS or linux. 2. Run: `pwsh` 3. `cargo add clap clap-complete -F clap/derivbe,clap-complete/unstable-dynamic` 4. Run: `COMPLETE=$(which pwsh) cargo run`. This is documented as supported in source. The main idea anyway is to show that, on any platform including Windows (powershell.exe is supported but basically EOL; pwsh is the way forward and has been for many years), `pwsh` is the executable you should be checking. ### Actual Behaviour > error: unknown shell `pwsh`, expected one of `bash`, `elvish`, `fish`, `powershell`, `zsh` ### Expected Behaviour Works like it would if the executable were `powershell` or for any other shell like `bash`. ### Additional Context Based on <https://github.com/PowerShell/PowerShell/issues/12150> and my own testing, `SHELL` is unreliable. Seems only the initial shell sets it. Once set, it's never overwritten, which is partly why PowerShell closed it. Even if I start bash (default shell) then zsh on macOS, `SHELL` is still "bash". Some other programs detect the parent process tree up to ~10 processes or so using `sysinfo` or similar. ### Debug Output _No response_