Use 'yes' instead of 'while-echo' in tests/ui/process/process-sigpipe.rs except 'nto'
The `sh` of AIX prints a message about a broken pipe when using the `while-echo` command. It works as expected when using the `yes` command instead. `yes` was originally used in this test but was later replaced with `while-echo` because QNX Neutrino does not have `yes` ([Replace yes command by while-echo in test tests/ui/process/process-sigpipe.rs](https://github.com/rust-lang/rust/pull/109379)). This PR updates the test to use `while-echo` for QNX Neutrino while reverting to `yes` for other platforms.
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