fix: audit fix — avoid latent stdout pipe deadlock in run_command (v0.8.3)

Piping stdout without draining while blocking in child.wait() risks deadlock
if a command writes more than one OS pipe buffer (~64 KB on Linux). Current
callers (systemctl, niri msg, loginctl) stay well under that, but the
structure was fragile. stdout is now discarded; stderr continues to be
captured for error reporting.

Cargo.lock

@@ -616,7 +616,7 @@ dependencies = [
 
 [[package]]
 name = "moonset"
-version = "0.8.2"
+version = "0.8.3"
 dependencies = [
  "dirs",
  "gdk-pixbuf",