guest: Emit debian service's debug log into stderr

So, these logs go into journald.

Bug: 400904451
Test: build

Change-Id: Ia760a4daf3f29ae5a3d370165e87756147a55112
diff --git a/guest/shutdown_runner/src/main.rs b/guest/shutdown_runner/src/main.rs
index aee5915..397c205 100644
--- a/guest/shutdown_runner/src/main.rs
+++ b/guest/shutdown_runner/src/main.rs
@@ -34,6 +34,7 @@
 
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::builder().filter_level(log::LevelFilter::Debug).init();
     let args = Args::parse();
     let gateway_ip_addr = netdev::get_default_gateway()?.ipv4[0];