Obtain user from OS

[allenap]

At present, the user name is obtained from the USER environment variable:

https://github.com/allenap/pgdo/blob/e9c001bec09f419fb75ebdfc96f1dc555613ad06/pgdo-lib/src/cluster.rs#L298C1-L307

But this isn't always set, especially in containers. We could obtain this more reliably from the OS directly, e.g.:

use nix::unistd::{getuid, User};
let user: Option<User> = User::from_uid(getuid());
let name: Option<String> = user.map(|user| user.name);