Add SSHCA_CLI_DEBUG_TEST_MACHINE_ID env var

When running a debug build, the `sshca host sign` command will now check
the `SSHCA_CLI_DEBUG_TEST_MACHINE_ID` environment variable for the value
to use as the machine ID.  This is useful during development and
testing, where the real machine ID is inaccessible or otherwise
unavailable.

The `SSHCA_CLI_DEBUG_TEST_MACHINE_ID` environment variable is *NOT* used
at all in release builds.

src/main.rs

@@ -180,6 +180,13 @@ fn get_hostname() -> Option<String> {
 }
 
 fn get_machine_id() -> Option<Uuid> {
+    #[cfg(debug_assertions)]
+    if let Ok(v) = std::env::var("SSHCA_CLI_DEBUG_TEST_MACHINE_ID") {
+        if let Ok(u) = Uuid::parse_str(&v) {
+            return Some(u);
+        }
+    };
+
     match std::fs::read_to_string(RPI_SERIAL_PATH) {
         Ok(s) => match Uuid::parse_str(&format!(
             "{:0>32}",