[BUG] panic when piping another ion echo into echo

Running ion -c "echo test" | echo panics at src/lib/builtins/echo.rs:156 when flushing the locked stdout in the BufWriter:

thread 'main' panicked at 'already borrowed: BorrowMutError', src/libcore/result.rs:1084:5
stack backtrace:
   0: std::panicking::default_hook::{{closure}}
   1: std::panicking::rust_panic_with_hook
   2: std::panicking::continue_panic_fmt
   3: rust_begin_unwind
   4: core::panicking::panic_fmt
   5: core::result::unwrap_failed
   6: <std::io::stdio::Stdout as std::io::Write>::flush
   7: ion_shell::shell::Shell::install_signal_handler::sigpipe_handler
   8: _sigtramp
[2]    59222 abort      ( ion -c "echo test"; )

ion -c "echo test" | gecho (where gecho is GNU's coreutils echo) breaks as well, while ion -c "gecho test" | echo works fine.

(ion -c "echo test" | cat is fine too)