sys: Use safe wrapper for libc::isatty

sys: Use safe wrapper for libc::isatty

Use the `std::io::IsTerminal` trait introduced in Rust 1.70.

Internally it calls `libc::isatty`, see [1, 2]. Note that it switches
the comparison from `== 1` to `!= 0` which shouldn't make a difference
assuming that libc::isatty upholds the promises made in its man page.

The MSRV was set on the workspace to reflect this change.

[1] https://doc.rust-lang.org/src/std/io/stdio.rs.html#1079
[2] https://doc.rust-lang.org/src/std/sys/unix/io.rs.html#79

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>