nix: add clap completions

Cargo.lock

@@ -201,6 +201,15 @@ dependencies = [
  "terminal_size",
 ]
 
+[[package]]
+name = "clap_complete"
+version = "4.5.65"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "430b4dc2b5e3861848de79627b2bedc9f3342c7da5173a14eaa5d0f8dc18ae5d"
+dependencies = [
+ "clap",
+]
+
 [[package]]
 name = "clap_derive"
 version = "4.5.49"
@@ -815,6 +824,7 @@ dependencies = [
  "bytes",
  "chrono",
  "clap",
+ "clap_complete",
  "futures-util",
  "nix",
  "sd-notify",