initStderrWriter

fn initStderrWriter(context: ?*anyopaque) callconv(.c) void {
    const ev: *Evented = @ptrCast(@alignCast(context));
    const cancel_protection = swapCancelProtection(ev, .blocked);
    defer assert(swapCancelProtection(ev, cancel_protection) == .blocked);
    ev.scan_environ.once(ev, &scanEnviron);
    const NO_COLOR = ev.environ.exist.NO_COLOR;
    const CLICOLOR_FORCE = ev.environ.exist.CLICOLOR_FORCE;
    ev.stderr_mode = Io.Terminal.Mode.detect(
        ev.io(),
        ev.stderr_writer.file,
        NO_COLOR,
        CLICOLOR_FORCE,
    ) catch |err| switch (err) {
        error.Canceled => unreachable, // blocked
    };
}