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/Cargo.toml b/guest/shutdown_runner/Cargo.toml
index 0b44baa..92f8762 100644
--- a/guest/shutdown_runner/Cargo.toml
+++ b/guest/shutdown_runner/Cargo.toml
@@ -7,6 +7,7 @@
 [dependencies]
 anyhow = "1.0.94"
 clap = { version = "4.5.20", features = ["derive"] }
+env_logger = "0.11.5"
 log = "0.4.22"
 netdev = "0.31.0"
 prost = "0.13.3"