cli: Trim trailing null from RPi serial

The `/sys/firmware/devicetree/base/serial-number` pseudo-file has a
trailing null byte, which causes `Uuid::parse_srr` to fail.  This makes
it impossible to authenticate Raspberry Pi devices to the server.  The
trailing byte needs to be removed before attempting to parse the serial
number into a UUID to avoid this problem.

src/main.rs

@@ -181,7 +181,10 @@ fn get_hostname() -> Option<String> {
 
 fn get_machine_id() -> Option<Uuid> {
     match std::fs::read_to_string(RPI_SERIAL_PATH) {
-        Ok(s) => match Uuid::parse_str(&format!("{:0>32}", s)) {
+        Ok(s) => match Uuid::parse_str(&format!(
+            "{:0>32}",
+            s.trim_end_matches('\0')
+        )) {
             Ok(u) => return Some(u),
             Err(e) => {
                 debug!("Invalid UUID: {}", e);