Main: Notify systemd about being ready after writing to the output path via sd_notify

When running in a systemd service with the --idle flag we
want to have systemd's service dependency chain depend on
the templated secret file being written to the output path.

src/main.rs

@@ -6,6 +6,7 @@ use std::thread;
 use anyhow::Result;
 
 use clap::Parser;
+use sd_notify::NotifyState;
 use serde_json::{Map, Value};
 use url_escape;
 
@@ -98,6 +99,8 @@ fn main() -> Result<()> {
     fs::write(&output_path, output)
         .expect(format!("To be able to write to '{}'", output_path.to_string_lossy()).as_str());
 
+    sd_notify::notify(true, &[NotifyState::Ready])?;
+
     if idle {
         loop {
             thread::park();