Panic if no USER variable is set

[Mynacol]

Your code currently unconditionally tries to get the USER environment variable which results in a program abort. That happens if you nixos-rebuild --target-machine [somewhere].

Full log:

thread 'main' panicked at src/main.rs:18:10:
called `Option::unwrap()` on a `None` value
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[thefossguy]

Again, as I mentioned here, this is really intended to be executed in a systemd service that runs after nixos-upgrade.service.