Mark arti-bench with "publish = false"

This crate is only useful for development; earlier we published it,
but I believe that was in error.

crates/arti-bench/Cargo.toml

@@ -11,6 +11,10 @@ keywords = ["tor", "arti"]
 categories = ["asynchronous"]
 repository = "https://gitlab.torproject.org/tpo/core/arti.git/"
 
+# This crate is only useful for arti development;
+# earlier versions of it were published by accident.
+publish = false
+
 [dependencies]
 anyhow = "1.0.23"
 arti = { path = "../arti", version = "1.1.6" }