Truncate exit code to u8, even if the real code doesn't fit in u8.

This prevents JuliaLauncher from panicking if a user exits with an error code
which doesn't fit in u8.
This is already the behaviour of Julia on Linux, where `ccall(:jl_exit)` can be
called with Int32(-1), yet returns error code 255.

src/bin/julialauncher.rs

@@ -366,7 +366,8 @@ fn main() -> Result<std::process::ExitCode> {
  }
  }
 
- let client_status: u8 = client_status?.try_into().unwrap();
+ // We truncate to the lower 8 bytes, even though a Windows exit code is i32
+ let client_status: u8 = client_status? as u8;
 
  Ok(std::process::ExitCode::from(client_status))
 }