fix(warning): Fix an unused import warning of std::fs on macOS

[ken-matsui]

Building zellij on macOS shows a warning in zellij-server/src/os_input_output.rs:

warning: unused import: `std::fs`
 --> zellij-server/src/os_input_output.rs:7:5
  |
7 | use std::fs;
  |     ^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `zellij-server` (lib) generated 1 warning

std::fs is only used when target_os is Linux:

zellij/zellij-server/src/os_input_output.rs

Lines 364 to 367 in 03e62eb

	#[cfg(target_os = "linux")]
	fn get_cwd(&self, pid: Pid) -> Option<PathBuf> {
	fs::read_link(format!("/proc/{}/cwd", pid)).ok()
	}