Similar to this PR (rust-lang/rust#55014).

In unwrap_or the fallback value is evaluated before unwrap_or is called. Which means even though a serial_device arg was provided

./target/debug/chunker ~/plot_file.hpgl /dev/tty.usbserial-1410

The program still failed

detected multiple serial devices: [
    "/dev/tty.usbserial",
    "/dev/tty.usbserial-1410",
], please specify only one!

This PR switches the unwrap_or call to unwrap_or_else. Which will only evaluate the fallback block in the else case.